fixup! Implement event_finalize() and related functions to avoid certain deadlocks

event_free() now avoids crashing even when there are pending finalizers.

More work needed: it should *call* finalizers; there should be a
variant that doesn't.
Add some verbose notes to bufferevent unit tests
New EVENT_DEBUG_LOGS envvar to enable debug logging for unit tests
Use finalization feature so bufferevents can avoid deadlocks

Since the bufferevents' events are now EV_FINALIZE (name pending),
they won't deadlock.  To clean up properly, though, we must use the
finalization feature.

This patch also split bufferevent deallocation into an "unlink" step
that happens fast, and a "destruct" step that happens after
finalization.

More work is needed: there needs to be a way to specify a finalizer
for the bufferevent's argument itself.  Also, this finalizer business
makes lots of the reference counting we were doing unnecessary.

Also, more testing is needed.