simplified reserved handling logic
simplified reserved handling logic

I think old code also had notified changing race where during shutdown
it was possible to have concurrent release call and on_disconnect
callback from memcached.

New logic is much simpler and hopefully obviously correct which I
cannot say about previous code.

Now reserved flag is always about releasing connection's
engine_specific. And release_cookie and on_disconnect handling are as
unified as possible.

Change-Id: Ia41a83936b7893a1e1c7bc488774324351726fd7
Reviewed-on: http://review.couchbase.org/13891
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

couchbase/bucket_engine,couchbase/bucket_engine,membase/bucket_engine,membase/bucket_engine,couchbase/bucket_engine,membase/bucket_engine,couchbase/bucket_engine,membase/bucket_engine