Allow for multiple db_event_cancel() (concurrent or
self-cancel) prior to event_task wakeup.
In db_event_cancel(), immediate free() only if idle
(not queued or in progress). Otherwise, defer free()
to event task. Avoids need to immediately expunge
canceled event from queue. Allow event task to
process canceled events as normal (except no user_sub)
until npend==0.
fab8fd7102