We introduce a new set of synchronisation primitives ("query ready")
that are signaled when the server is ready to take queries, and make
spider initialisation use them instead of the primitives associated
with server start.

During spider initialisation, if the server is query ready, proceed
with the initialisation synchronously (requiring the sql
service). Otherwise, leave the init to the first sts bg thread as
usual.

When creating a spider handler before the spider init is done, if the
server is not query ready, just give up and return NULL. If the server
is query ready, then wait for a specific amount of time (5s) for the
spider init to be done, and on timeout give up and return NULL.

How this fixes / prevents various init bugs:

MDEV-27233 (spider installed and used in init file). Before -
spider_create_handler() called by read_init_file() waits for bg thread
to finish init, whereas the bg thread is waiting for server
start. After - spider init is run synchronously because the server is
query ready when read_init_file(). No funny business with concurrency.

MDEV-29904 (spider loaded with plugin-load-add). Before (before the
original patch to MDEV-27233 was reverted) - fully synchronous spider
init fails due to dependencies on Aria / acl init. After - spider init
is run async, and as soon as the server is query ready, the init
proceeds and succeeds.

MDEV-22979 (mysqld --bootstrap --plugin-load-add=ha_spider). Before -
bootstrap queries causes call to spider_create_handler(), which waits
for bg thread to finish init, whereas bg thread waits for server
start. After - spider init is run async, waiting for server to be
query ready. spider_create_hander() notices bg thread still trying to
init spider, and server is not query ready so it gives up.

MDEV-28218 (spider is loaded in a query after server start,
immediately before another query dropping a spider table). Before - bg
thread is created to init spider async, while the server proceeds to
the next query that tries to drop a spider system table. race
condition causes the situation where the server holds the mdl lock to
the table and waits for the bg thread in spider_create_handler(), and
the bg thread waiting for the mdl lock to create the table. After -
since server has already started, spider init is run synchronously,
thus no concurrency issues. Same applies to the variant of MDEV-28218
represented by mdev_28218_init_file where the queries are in an init
file.

Another variant of MDEV-28218 represented by mdev_28218_mixed loads
spider with plugin-load-add and executes the drop table query in an
init_file. Spider init is async because the server was not query
ready. As soon as the the server becomes query ready the server tries
to execute the drop table query. It waits on the init in
spider_create_handler(), but because of the deadlock it times out
after 5 seconds. The drop table query did not find any table, but
spider init then proceeds as it finally acquires the mdl lock, and
creates the table. The moral of the story is to advise users not to
mix plugin-load-add=ha_spider with init_file containing queries
related to spider system tables. Queries using spider engine to create
tables are still ok because there's no deadlock.

MDEV-30370 (mysqld --wsrep-recover --plugin-load-add=spider). The
timed wait in the patch for MDEV-30370 still applies, so it still
passes.

More on the design:

Compared to the server start primitives, the query ready primitives
are signaled earlier ("query ready point") to avoid unnecessary wait
and deadlocks.

The spider initialisation does not only require other plugins like
Aria to be initialised first, but also acl init (due to its creation
of udfs) as well as the creation of the `mysql` database (due to its
system tables being in this database). Thus the point where it can go
ahead with the initialisation should be after the bootstrap.

By the time the server can accept queries by reading from init files,
it can definitely accept all spider init queries. So the point of
spider initialisation can be before this point.

Looking at the lines between boostrap and reading init file, the most
natural position is just before the call to read_init_file(), where we
can also give the primitives a most meaningful name "query ready".