MB-12574 Adapt according to the forestdb API changes

ForestDB has few API changes to introduce support for multiple
kvstores within a file. This commit migrates from old single kvstore
per file APIs to multi-kvstore per file APIs.

Change-Id: I82cbe6ca8fec62f278b7a2e7d97bd8a48a30dc90
Reviewed-on: http://review.couchbase.org/43017
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>