Rework Clean Task, merge it with compaction

[jankotek]

Clean (destroy all but last N snapshots) right now runs in foreground; user code has to call it and wait until clean up finishes.

However full cleanup moves lot of data and should be ran as part of compaction in background. Also cleanup should be incremental.

So:

• rework compaction so that some snapshots are 'valid' and 'invalid'
  • invalid shapshots are compacted and destroyed
  • rework compaction code to preserve valid snapshots
• cleanup method only invalidates snapshots, and returns immediately
• heurestic in compaction to merge Shard every N Updates.
  • Log can have multiple Merged updates in linked-list?
  • based on file size, rather than number of updates?

[jankotek]

Fixed in 0.2 release