[Designing] Read snapshot before it get replayed

[yinweiishere]

Snapshot will be created by inserting a snapshot type journal and persist snapshot metadata in DR server.
There will be a period between creation and snapshot journal to be replayed (how to ensure the ordering?).
Issues:

1. How to define snapshot created?
   1.1 having persisted both journal and metadata;
   1.2 snapshot journal being replayed;
   1.3 snapshot journal has been replicated to remote;
   1.4 snapshot journal has been replayed in remote site;

2. If we define snapshot created as 1.1, remote snapshot created as 1.3, how to read/diff/roll back snapshot if it hasn't been replayed yet?

3. For remote snapshot, we need make the replication pair volume with a snapshot as well in cinder, but with the same snapshot id as the local one: need rethink the replication pair design.