raft: Only call stableTo when we have ready entries or a snapshot.

The first Ready after RestartNode (with no snapshot) will have no
unstable entries, so we don't have the correct prevLastUnstablei
when Advance is called. This would cause raftLog.unstable to move
backwards and previously-stable entries would be returned to
the application again.

This should have been caught by the "unexpected Ready" portion of
TestNodeRestart, but it went unnoticed because the Node's goroutine
takes some time to read from advancec and prepare the write to read to
readyc. Added a small (1ms) delay to all such tests to ensure that the
goroutine has time to enter its select wait.
Not easily. `stableTo(i)` means `unstable = i + 1`, so we'd need to subtract 1 here, but that fails when unstable is zero. If we used int64 instead of uint64 then we could use -1 as a sentinel value.