Rework session to console relationship
Rework session to console relationship

Previously, was using counters to track the relation, also had distinct tracking of users versus
callbacks.  Unify the callback and user into a single 'session' attach and then use the size
of the set of sessions and their declared users rather than trying to maintain a counter on the side.

This change simplifies the relationship, changes away the logging and clientcount counter for
a more robust strategy, and paves the way for the dependent ShellHandler to terminate connected
sessions when the shell session dies.

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