support SQL semantics for SELECT ... WHERE ... ORDER BY ... LIMIT

* switch from returning k nearest neighbors to returning
  as many as needed, in k-neighbor chunks, with increasing distance
* make search_layer() skips nodes that are closer than a threshold
* read_next keeps a search context - list of k found nodes,
  threshold, ctx, etc.
* when the list of found nodes is exhausted, it repeats the search
  starting from last found nodes and a threshold
* search context kepts ctx->refcount incremented, so ctx won't go away
* but commit_lock is unlocked between calls, so InnoDB can modify the table
* use ctx version to detect that, switch to MHNSW_Trx when it happens

bugfix:
* use the correct lock in ha_external_lock() for the graph table
* InnoDB didn't reset locks on ha_external_lock(F_UNLCK) and previous
  LOCK_X leaked into the next statement

renames

MDEV-35029 ASAN errors in Lex_ident<Compare_ident_ci>::is_valid_ident upon DDL on table with vector index

in ALTER TABLE or CREATE TABLE LIKE, a create a copy of key->option_list,
because it can be extended later on the thd->mem_root, so it has
to be a copy

MDEV-35033 LeakSanitizer errors in my_malloc / safe_mutex_lazy_init_deadlock_detection / MHNSW_Context::alloc_node and alike

ctx wasn't released on errors

MDEV-35020 After a failed attempt to create vector index temporary file remains and prevents further operation

MDEV-35035 Assertion failure in ha_blackhole::position upon INSERT into blackhole table with vector index

let's allow ::position() and ::rnd_pos() in blackhole.
::position() can be called directly after insert, it doesn't need
a search to happen, so it's possible.

::rnd_pos() can be called with a value that ::position() produced,
so, possible too.

MDEV-35037 Invalid (old?) table or database name 't#i#00' upon creating RocksDB table with vector index

disallow it, for now

also fixes

MDEV-35036 Assertion failure in myrocks::ha_rocksdb::position upon INSERT into RocksDB table with vector index

MDEV-35038 Server crash in Index_statistics::get_avg_frequency upon EITS collection for vector index

don't collect eits for vector indexes

MDEV-35039 Number of indexes inside InnoDB differs from that defined in MariaDB after altering table with vector key

don't show table->s->total_keys to engine in inplace alter

* hlindexes cannot be extended with pk
* hlindexes cannot be covering

add missing casts to float4store/float8store for bigendian.
fix a typo in float4store() usage
remove unnecessary one-byte-at-time appends

make generosity depend on

1. M. Keep small M's fast, increase generosity for larger M's to get
   better recall.

2. distance. Keep generosity small when vectors are far from the
   target, increase generosity when the search gets closer. This
   allows to examine more relevant vectors but doesn't waste time
   examining irrelevant vectors. Particularly important with cosine
   metric when the distance is bounded

... until a few bugs that cause server crash are fixed.

also: renames, s/const/constexpr/ for consistency

Replaced obscure FRM_ONLY, NO_FRM_RENAME, NO_HA_TABLE, NO_PAR_TABLE with
straightforward explicit flags:

QRMT_FRM - [re]moves .frm
QRMT_PAR - [re]moves .par
QRMT_HANDLER - calls ha_delete_table()/ha_rename_table() and [re]moves
               high-level indexes
QRMT_DEFAULT - same as QRMT_FRM | QRMT_HANDLER, which is regular table
               drop/rename.

quick_rm_table() expects .frm to exist when it removes high-level indexes.
For cases like ALTER TABLE t1 RENAME TO t2, ENGINE=other_engine .frm was
removed earlier.

Another option would be removing high-level indexes explicitly before the
first quick_rm_table() and skipping high-level indexes for subsequent
quick_rm_table(NO_FRM_RENAME).

But this suggested order may also help with ddl log recovery. That is
if we crash before high-level indexes are removed, .frm is going to
exist.

Disable non-copy ALTER algorithms when VECTOR index is affected. Engines
are not supposed to handle high-level indexes anyway.

Also fixed misbehaving IF [NOT] EXISTS variants.

Fixes for ALTER TABLE ... ADD/DROP COLUMN, ALGORITHM=COPY.

Let quick_rm_table() remove high-level indexes along with original table.

Avoid locking uninitialized LOCK_share for INTERNAL_TMP_TABLEs.

Don't enable bulk insert when altering a table containing vector index.
InnoDB can't handle situation when bulk insert is enabled for one table
but disabled for another. We can't do bulk insert on vector index as it
does table updates currently.

and create a parent Item_func_vec_distance_common class

into a separate transaction_participant structure

handlerton inherits it, so handlerton itself doesn't change.
but entities that only need to participate in a transaction,
like binlog or online alter log, use a transaction_participant
and no longer need to pretend to be a full-blown but invisible
storage engine which doesn't support create table.

remove unused methods, reorder methods, add comments

Rename high-level indexes along with a table.

* fix the truncate-by-handler variant, used by InnoDB
* test that insert works after truncate, meaning graph table was emptied
* test that the vector index size is zero after truncate in MyISAM

This patch fixes only TRUNCATE by recreate variant, there seem to be no
reasonable engine that uses TRUNCATE by handler method for testing.

Reset index_cinfo so that mi_create is not confused by garbage passed via
index_file_name and sets MY_DELETE_OLD flag.

Review question: can we add a test case to make sure VECTOR index is empty
indeed?

This commit introduces two utility functions meant to make working with
vectors simpler.

Vec_ToText converts a binary vector into a json array of numbers
(floats).
Vec_FromText takes in a json array of numbers and converts it into a
little-endian IEEE float sequence of bytes (4 bytes per float).

This method will write out a float to a String object, keeping the
charset of the original string.

Also have Float::to_string make use of String::append_float

introduced a generosity factor that makes the search less greedy.
it dramatically improves the recall by making the search a bit slower
(for the same recall one can use half the M and smaller ef).

had to add Queue::safe_push() method that removes one of the
furthest elements (not necessarily the furthest) in the queue
to keep it from overflowing.

to return only as many elements as needed, the caller no longer needs to
overallocate result arrays for throwaway nodes