MB-11036: Support for session cas validation

- Provide API for ns_server to set the session_cas.
- Provide API for ns_server to get current session_cas.
- Provide an API for ep_engine to validate the session_cas
for every request from ns_server
- Validate the session_cas before creating and deleting a
bucket instance.

Change-Id: Iea8ff75c4d19b312c538b9c66eb3fedca7cbfd38
Reviewed-on: http://review.couchbase.org/36735
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>