Pool::mem_free(): Poison the freed memory. Assert that it was
fully initialized, because the reuse of trx_t objects will
assume that the objects were previously initialized.

Pool::~Pool(), Pool::get(): Unpoison the allocated memory,
and mark it initialized.

trx_free(): After invoking Pool::mem_free(), unpoison
trx_t::mutex and trx_t::undo_mutex, because MutexMonitor
will access these even for freed trx_t objects.