Consider (optionally?) compacting in the background

[msm-code]

Consider (optionally?) compacting in the background, or measure the impact compacting has on database performnace.

In the current version compacting is (almost) always a good idea for performance, and in theory we recommend everyone to compact their database, but ursadb won't use idle time to compact the files. Consider doing that in the background when the system is idle (or move this issue to mquery if that should be done by the clients).