Generic mechaism for "post-loop" callbacks

We've been labeling some events as happening "outside the event
loop", to avoid Libevent starvation.  This patch provides a cleaner
mechanism to avoid that starvation.

For background, the problem here is that Libevent only scans for new
events once it has run all its active callbacks.  So if the
callbacks keep activating new callbacks, they could potentially
starve Libevent indefinitely and keep it from ever checking for
timed, socket, or signal events.

To solve this, we add the ability to label some events as
"post-loop".  The rule for a "post-loop" event is that any events
_it_ activates can only be run after libevent has re-scanned for new
events at least once.

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