Cache store generic

[aakoshh]

A generic version of #897 using the storage abstractions instead of rocksdb directly.

The PR isn't finished, I just wanted to see what were the problems that stopped @fridrik01 from using the KVStore. I won't lie it's a lot of generics; in a way the syncer is the one of the worst places to introduce this kind of stuff because it's layers and layers of indirection.

I tried to alleviate it a little by removing the S parameter from App. I imagine it might help to try to wrap things up into abstractions that do a specific cache functionality, rather than having to pass around a KVStore parameter together with all the things that it needs to be able to serialize. But I wanted to do this to show the way it can be done.

Had to refactor STM a bit in aakoshh/async-stm-rs@113be84 to get around a compilation issue. This now demonstrates how to use RocksDB transactions together with STM ones. There might be ways to refactor the panicky commit/rollback, just need to have another long look at how they are used.