SPY-151: Interrupt pending AuthThreads during shutdown.

This changeset fixes hanging auth threads while shutting down the
connection.

Change-Id: I386fea0da2d06b655c7e3cd0d843761d59ee7e28
Reviewed-on: http://review.couchbase.org/31828
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>