Before this commit, it will return wrong create index, mod index.
It lets findGeneration return error when rev is at the gap of two
generations. This leads to the change of compact() code.
Snapshot takes an io.Writer and writes the entire backend data to
the given writer. Snapshot writes a consistent view and does not
block other storage operations.
Restore restores the in-memory states (index and book keeping) of
the storage from the backend data.
