db: download compactions are being counted towards the compaction concurrency

[sumeerbhola]

We add download compactions to the inProgress compaction map here https://github.com/cockroachdb/pebble/blob/master/download.go#L439 (we also add flushes to this map). Download compactions have their own concurrency limit specified by Options.MaxConcurrentDownloads(). For this reason these download compactions don't increment DB.mu.compact.compactingCount, and instead increment DB.mu.compact.downloadingCount. But in compactionPickerByScore.pickAuto we are using the length of the in-progress compactions (which excludes flushes but includes download compactions) to decide whether we are allowed to pick another compaction.

pebble/compaction_picker.go

Line 1172 in 1157615

if n := len(env.inProgressCompactions); n > 0 {

This is incorrect -- download compactions should be excluded from this length calculation.

Jira issue: PEBBLE-336