Only call active_sessions GC if the worker has created a session since last time.
gc_period of zero will now disable the GC
Add config option to disable active session tracking.
Merge branch 'develop' into feature/issue-295-less-aggressive-session-manager-polling
Docstring updates.
Count active sessions by listing keys instead of keeping track in a set.
Remove stray line left over from prior commit.
Merge branch 'feature/issue-295-less-aggressive-session-manager-polling' into develop