sstable: parallel / pipelined decompression during compaction

[nicktrav]

This issue was spun out of #599, tracking the decompression component. The compression piece has already been implemented.

When looking at CPU profiles, block decompression during reads, and compressing writes, show up significantly. On the read side, it would be nice for that decompression to take place asynchronously via some sort of read-ahead mechanism.

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it
in 10 days to keep the issue queue tidy. Thank you for your
contribution to Pebble!