SegmentRevisionGC MBean should report more detailed gc status information

Regarding this, the current "Status" is showing the last log info. This is useful, but it would also be interesting to expose the real-time status. For monitoring it would be useful to know exactly in which phase we are, e.g. a field showing on of the following:

• idle
• estimation
• compaction
• compaction-retry-1
• compaction-retry-2
• compaction-forcecompact
• cleanup