journals: implement rudimentary read buffers

This adds eight 8KiB "fixed" buffers per opened journal,
recycled in a basic LRU fashion.

Any read 8KiB or smaller passes through this cache, simply
memcpy()d from the buffer when already resident, or upsized to an
8KiB read when absent, to then be memcpy()d out of the populated
buffer when the read into the buffer completes.

Any read larger than 8KiB bypasses the buffers to be read
directly into the provided destination via iou as if the cache
weren't present at all.

0 files changed, 0 insertions, 0 deletions