Fix bugs in snapshot move (#2440)

- Fix a bug with snapshot move, due to which the move would block forever. In particular, if a key-value didn't have a Version, then `txn.CommitAt` failed and didn't ever call `wg.Done()`. This caused `wg.Wait()` to block forever, hence keep the snapshot retrieval stuck forever.
- Fix a nil pointer dereference issue in `pool.shutdown`.
- Fix a nil pointer dereference issue with member connection.

This PR fixes #2410 .

Potential improvement:
- Make the predicate move concurrent, so we can quickly transfer large amounts of data in a short span of time. Can't prove its utility right now, but this should be useful in large data sets.