Schedule the background compactors to run.

This commit introduces three background compactors, which compact
sparse samples together.

1. Older than five minutes is grouped together into chunks of 50 every 30
   minutes.

2. Older than 60 minutes is grouped together into chunks of 250 every 50
   minutes.

3. Older than one day is grouped together into chunks of 5000 every 70
   minutes.
Merge pull request #228 from prometheus/feature/storage/compaction

Schedule the background compactors to run.