db: flesh out CompactionInfo.Reason

[petermattis]

Fill in CompactionInfo.Reason with the kind of compaction: default,
move, delete-only, elision-only, or read.

Fixes #1142

[cockroach-teamcity]

This change is 

[petermattis]

We don't have good test coverage of this, but I did run pebble bench ycsb -v --workload=C ... and verified that we're seeing compacting(default), compacting(move), and compacting(read).

[sumeerbhola]

Reviewed 5 of 5 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @petermattis)

---

metrics.go, line 129 at r1 (raw file):

	Compact struct {
		// The total number of compactions, and per-compaction type counts.

why does this say "per-compaction type counts" -- we don't seem to have that.

[petermattis]

Reviewable status: 4 of 5 files reviewed, 1 unresolved discussion (waiting on @sumeerbhola)

---

metrics.go, line 129 at r1 (raw file):

Previously, sumeerbhola wrote…

why does this say "per-compaction type counts" -- we don't seem to have that.

Oops. I started down the path of implementing #1145, then reversed course. This was a left-over from the detour.

[petermattis]

TFTR!