Gets rid of a race where unpause+pause fired in a quick succession would
miss the event_base_loopbreak() call.

Reuse the connection walking facility in timeout management.

This simplifies port parsing in the end. Also pass the url to
ldap_open_listener in anticipation of incremental listener config.

Unregistration is a hack and we shoould either make the subsystems into
an entry (if monitor allows subentry generation) or implement subsystem
unregistration in back-monitor.

An operation is rejected iff it has to be dropped before we can find an
upstream for it (unless we handle it ourselves, that is). At that point
it is failed unless completed successfully.

This makes a difference for multi-stage binds which alternate between
'failed' (we are waiting on a server response) and 'completed' (server
did what we asked them to, waiting on client to continue).