CBIDXT-33 Structures to represent Slice and Snapshot

Patchset3:
- More comments for slice status
- Change RW Lock to RLock for snapshot container
  for get methods

Change-Id: I9dc8f2598768ce729a903190c0a6edbc0c7c5806
Reviewed-on: http://review.couchbase.org/38069
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>