The DeadlockChecker expects to be able to freeze the waits-for graph.
Hence, it is best executed somewhere where we are not holding any
additional mutexes.

lock_wait(): Defer the deadlock check to this function, instead
of executing it in lock_rec_enqueue_waiting(), lock_table_enqueue_waiting().

DeadlockChecker::trx_rollback(): Merge with the only caller,
check_and_resolve().

LockMutexGuard: RAII accessor for lock_sys.mutex.

lock_sys.deadlocks: Replaces lock_deadlock_found.

trx_t: Clean up some comments.