Use state snapshots to bootstrap partition processor on start-up

[pcholakov]

On start up, the partition processor needs to check in the snapshot store whether there exists a snapshot that is newer than its own local data. If that's true, then it needs to download it and load it into the local state store. To avoid frequent checks to the snapshot store (possible optimization), it could also try to resume with its local data only and only load the data from the snapshot store if it encounters a trim gap.