Problem: memiavl don't support async snapshotting #948
Description
we have basic support with WriteSnapshot, but to support async snapshotting, we need:
- take a in-memory snapshot
- this means we need to support copy-on-write on the
MemNodes.
- this means we need to support copy-on-write on the
- start a go-routine which do the actual snapshot work
- after the new snapshot created successfully, catch-up with later change sets, and switch.