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A full read cycle can take a very long time if the limits are set too
high.

Similar to the algorithm presented in
https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf

Not completely lock-free at the moment. Also the problems with epoch
based memory reclamation are still present - a thread actively observing
an epoch getting stuck will prevent LloadConnections and LloadOperations
being freed, potentially running out of memory.

LDAP_UNAVAILABLE signals "the server is shutting down or a subsystem
necessary to complete the operation is offline", so intelligent clients
tend to infer the connection will not be usable any more, which is not
the case here.

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.