CBIDXT-19 ForestDB implementation for Slice and Snapshot

Change-Id: I0f93072ec77a944258cf66e69410fd625bdf4371
Reviewed-on: http://review.couchbase.org/38143
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>