MB-11071 Support multiple KV instances

- Add three new main operations: open (create), close, remove of KV
instance in a DB file handle.

- All other operations (get, set, rollback, snapshot, etc.) are shared
with the original APIs.

- Same as snapshot, KV instance uses the fdb_handle structure, but
commit and compaction can be invoked only using the DB file handle.

Change-Id: Iad571745c57053d46130be584c5213e98baa9b9f