SERVER-13961 Add capability to register and discover OperationContexts

This replaces the need to iterate through the list of registered Clients
for diagnostics/reporting purposes. Also moves some of the per-client
information to be under OperationContext.

This is in preparation for removing LockState from TLS.
SERVER-13961 Remove TLS LockState accesses from repl
Merge remote-tracking branch 'upstream/master'