consistentWatchableStore maintains an index that is always consistent
with the latest txn. The index could be used to indicate the progress
of the store so far when recovery.
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.
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.
