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.
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.
Introduces pinned operations. When SASL bind finishes, we might still
have to maintain a link between the client an an upstream for future
bind operations if we got a SASL Bind in Progress result code. We zero
out the msgids and remember a server-unique identifer on the client and
the relevant operation that lets us retrieve that link again. This
operation is reclaimed just like anything else when connections drop.
Hopefully, this should work for LDAP TXN and VC Exop support with SASL
later as well since it allows for many-to-many links to exist.
