Log Back-End #23

could eventually borrow heavily from https://github.com/spacejam/rust-reactive-log but for now we can probably just chuck messages into rocksdb with the key collection-id + txid

• protobufs
• storage layer
• retention policy (will need to do this in a collection generic way, as objects and kv will possibly want to have specifiable history retention to support reliable mutation subscription and replay)
• server client request handling logic