The problem with this member is that it can have the same value as
the current session's THD, it can have the value of another session's
THD, or it can be NULL, in various stages of execution.
For internal temporary tables, the problem is that for temporary tables
associated with triggers, the trigger may be passed on from one
session to another, and there is no well defined point where this
transition happens.
To tighten the use, we will instead set the in_use member to NULL
when the internal temporary table is closed and assign the current
session's THD pointer to it when the temporary table is used by
another session.
This is all well, but there is also much use of the in_use throughout
server code, because it is a convenient way of accessing the THD
pointer when it is not passed as a function argument, or by other
other means. We will therefore start recommending against this
practice and substitute old use by either a passed THD argument
or the thread-local variable current_thd.

Storage engines still need in_use, so no change is made here.
We also use in_use to check for MDL lock waits, so this use stays.

For storage engines, it might be an idea to pass the THD pointer
though the HANDLER object instead. It should make a more stable
interface than TABLE::in_use.

The THD argument to close_tmp_table() has also been eliminated.
This also implicated removing server stage: deleting temporary table.

Reviewed by: Catalin Besleaga <catalin.besleaga@oracle.com>