Snapshot support in ForestDB

A snapshot can be created explicitly by calling snapshot API and only allows us to perform read-only operations. A snapshot is destroyed if its corresponding database handle is closed.

Creating a snapshot instance should NOT corrupt the WAL index used by the writer. Otherwise, it will overwrite the current WAL entries with the stale WAL docs from the old header of the snapshot.

Note that we maintain a single WAL index per file (NOT per handle). To address this issue, we need to restore the read-only WAL index separately for a given snapshot.