1. ER_KEY_NOT_FOUND
general replcation problem, already fixed earlier.
test added.

2. ER_LOCK_WAIT_TIMEOUT
This is a long unique specific problem.

Sometimes, lookup_handler is created for to->file. To properly free it,
ha_reset should be called. It is usually done by calling
close_thread_table, but ALTER TABLE makes it differently. Hence, a single
ha_reset call is added to mysql_alter_table.

Also, event_mem_root is removed. Normally, no per-event data should be
allocated on thd->mem_root, that would mean a leak. And otherwise,
lookup_handler is lazily allocated, but its lifetime matches statement,
not event.