Adds serialization with compression and checksum for WAL buffer segments.
This required a weird structure where the flatbuffer bytes of ReplicatedWrite were kept as a raw payload. I did this because otherwise each of the replicated writes would have been rebuilt in the segment.
The other thing that isn't ideal is that deserializing a segment actually marshals it into a Rust struct as opposed to keeping the entire thing as raw flatbuffers. We could update this later to have a concept of an open segment (regular rust stuct) and closed segments that are just the flatbuffers.
This splits the cluster package out into server and buffer modules. The WAL buffer is in-memory and split into segments. Follow on commits will implement it in the server and add persistence to object storage.
Paul Dix
Andrew Lamb