This has the (somewhat nice) side effect that it shrinks the in-mem
catalog a bit as well because nw `ParquetChunk` is a bit smaller making
the chunk stage enum smaller as well.
The entire persistence windows data structures (including the
checkpoints) have nothing to do with the mutable buffer per se. So lets
move them into their own crate. This also makes `parquet_file` not
longer depend on `mutable_buffer`.
Raphael Taylor-Davies
Marco Neumann
Andrew Lamb