When using Snapshot function, it is expected:
1. know the size of snapshot before writing data
2. split snapshot-ready phase and write-data phase. so we could cut
snapshot first and write data later.
Update its interface to fit the requirement of etcdserver.
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.
