Add support for callbacks in the Schedd.query method. #4111
Allow DC sockets to be registered for write.
Allow CEDAR to indicate when there is a backlog for sending packets to the network.  Use this to implement a putClassAd variant which can detect backlog.
Start to make condor_q responses non-blocking.
Initial implementation of server-side for a non-blocking condor_q protocol. #4111
Implement new query command in python. #4111
Clean up non-blocking mode.

Rename python binding to 'xquery', to match python naming conventions. #4111

Change non-blocking interfaces to be more difficultto abuse.

Add python unit tests and documentation.

Have non-blocking queue return to DC periodically, even if we don't
have to due to blocking.
Get border conditions correct for hash table and classad_log iterators.
Implement non-blocking condor_q. #4111

Have the condor_q CLI utilize the non-blocking protocol starting
with HTCondor 8.1.4.
Update version history for non-blocking condor_q query protocol.
Fix ownership issues which cause segfaults with some condor_q flags. #4111
Implement non-blocking reads for ReliSocks.
Fix memory usage when receiving packet. #4111
Merge branch 'non_blocking_condor_q' #4111