If backend is closed, the operations on backend in compact
goroutine will panic. So this PR waits for compact goroutine to exit
before close backend.
This fixes the TestWorkflow failure too.
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.
