There was a race condition where the connection of the
victim of a KILL statement is disconnected while the
KILL statement is executing.

As a side effect of this fix, we will make XA PREPARE
transactions immune to KILL statements.

Starting with MariaDB 10.2, we have a pool of trx_t objects.
trx_free() would only free memory to the pool. We poison the
contents of freed objects in the pool in order to catch misuse.

trx_free(): Unpoison also trx->mysql_thd and trx->state.
This is to counter the poisoning of *trx in trx_pools->mem_free().
Unpoison only on AddressSanitizer or Valgrind, but not on MemorySanitizer.

Pool: Unpoison allocated objects only on AddressSanitizer or
Valgrind, but not on MemorySanitizer.

innobase_kill_query(): Properly protect trx, acquiring also
trx_sys_t::mutex and checking trx_t::mysql_thd and trx_t::state.