Fix some race conditions in persistent events and event_reinit

Fix some race conditions in persistent events and event_reinit

I found these by adding an EVENT_BASE_ASSERT_LOCKED() call to most
of the functions in event.c that can only be called while holding
the lock.

event_reinit() never grabbed the lock, but it needed to.

event_persist_closure accessed the base to call event_add_internal()
and gettime() when its caller had already dropped the lock.

event_pending() called gettime() without grabbing the lock.