MDS: use distinct thread pools for background flushes and compaction

[redlicha]

https://github.com/facebook/rocksdb/wiki/RocksDB-Basics#avoiding-stalls
https://github.com/facebook/rocksdb/wiki/Write-Stalls

[wimpers]

@redlicha can you add an estimate of how much work this would be and when you think it should be picked up? is this urgent or can we put it on the roadmap.

[redlicha]

Urgency: it was hypothesized to be the cause for #350 but eventually #363 turned out to be the reason.
Reading the above links it still looks like a potential bottleneck that warrants further investigation, so I think "roadmap"ing it is ok. Ballpark guesstimate: 1- 2 days.