perf: consider atomic compaction units in elision-only compaction heuristic

[jbowens]

If a file that is eligible for an elision-only compaction contains a single user key, we may need to pull adjacent files into the compaction to ensure we always compact atomic compaction units together. This may dramatically change the space/write IO tradeoff, resulting in rewriting substantially more data than we drop.

This abbreviated log line was taken from a node affected by #1070.

vendor/github.com/cockroachdb/pebble/compaction.go:1410 ⋮ [nXXX] [JOB XXXXX] compacted L6 [046035] (168 M) + L6 [] (0 B) -> L6 [046647 046648 046649 046650 046651 046652 046653 046654 046655 046656 046657 046658 046659 046660 046661 046662 046663 046664 046667 046669 046671 046672 046673 046674 046675 046676 046677 046678 046679 046680 046681 046682 046683 046684 046685 046686 046687 046688 046689 046690 046691 046692] (166 M), in 3.5s, output rate 48 M/s

[jbowens]

This was actually #1097.