Disable per-thread caching of buffers for stored fields

Stored fields currently cache buffers that are used to decompress data. However with one buffer per thread per segment, these buffers can start using non-negligible amounts of data.

I think we should keep them for merges, but either remove them completely for regular reads, or sharing them across all segments. As I'd like to avoid relying on global threadlocals, I'm leaning towards removing them.