Rollback attempted to dereference DB_ROLL_PTR=0, which cannot possibly
be a valid undo log pointer. A safer canonical value would be
roll_ptr_t(1) << ROLL_PTR_INSERT_FLAG_POS
which is what was chosen in MDEV-12288, corresponding to reset_trx_id.

No deterministic test case for the bug was found. The simplest test
cases may be related to MDEV-11415, which suppresses undo logging for
ALGORITHM=COPY operations. In those operations, in the spirit of
MDEV-12288, we should actually have written reset_trx_id instead of
using the transaction identifier of the current transaction
(and a bogus value of DB_ROLL_PTR=0). However, thanks to MySQL Bug#28432
which I had fixed in MySQL 5.6.8 as part of WL#6255, access to the
rebuilt table by earlier-started transactions should actually have been
refused with ER_TABLE_DEF_CHANGED.

reset_trx_id: Move the definition to data0type.cc and the declaration
to data0type.h.

btr_cur_ins_lock_and_undo(): When undo logging is disabled, use the
safe value that corresponds to reset_trx_id.

btr_cur_optimistic_insert(): Validate the DB_TRX_ID,DB_ROLL_PTR before
inserting into a clustered index leaf page.

ins_node_t::sys_buf[]: Replaces row_id_buf and trx_id_buf and some
heap usage.

row_ins_alloc_sys_fields(): Init ins_node_t::sys_buf[] to reset_trx_id.

row_ins_buf(): Only if undo logging is enabled, copy trx->id
to node->sys_buf. Otherwise, rely on the initialization in
row_ins_alloc_sys_fields().

row_purge_reset_trx_id(): Invoke mlog_write_string() with reset_trx_id
directly. (No functional change.)

trx_undo_page_report_modify(): Assert that the DB_ROLL_PTR is not 0.

trx_undo_get_undo_rec_low(): Assert that the roll_ptr is valid before
trying to dereference it.

dict_index_t::is_primary(): Check if the index is the primary key.

PageConverter::adjust_cluster_record(): Fix
MDEV-15249 Crash in MVCC read after IMPORT TABLESPACE
by resetting the system fields to reset_trx_id instead of writing
the current transaction ID (which will be committed at the
end of the IMPORT TABLESPACE) and DB_ROLL_PTR=0.
This can partially be viewed as a follow-up fix of MDEV-12288,
because IMPORT should already then have written
DB_TRX_ID=0 and DB_ROLL_PTR=1<<55 to prevent unnecessary
DB_TRX_ID lookups in subsequent accesses to the table.