[WIP] Blocking command support

[TalZaccai]

• Adding support for blocking commands in Garnet (for List and SortedSet objects)
• Implementing logic for BRPOP/BLPOP commands

Still remaining to address in this PR:

• Block processing of further commands once a blocking command has been called
• Stop the main loop in CollectionItemBroker if there are no active observers
• Periodically clean up "zombie" observers (observers whose resp session has disposed but are still on the key's observer queue)
• Remove keys whose observer queues are empty
• Add more test cases

[badrishc]

Wait on a SemaphoreSlim here, that can be canceled (using a cancellation token) if the socket is killed or the session dies for some reason (i.e., session Dispose gets called).