Slots & commitments

[polachok]

1. We currently store & report the latest slot seen in either validator response or received in pubsub (per commitment level). This leads to slot in our responses lagging behind validator if there're no changes for cached accounts.

We should find out how to get the latest slot for commitment via pubsub (preferably).

2.If we have account A cached with slot = N, commitment = Processed, and later we receive an update for account B with slot N+1, commitment = Finalized, it seems like it means that slot N is also Finalized, which in turn, means that we can return account A's data for a request with Finalized commitment requirement.