Pass KeyExtent to tablet for commit RPC

[keith-turner]

The coordinator knows of the extent and UUID when it makes the RPC to commit a compaction. However it only passes the UUID. It could also pass the Extent to allow for another sanity check on the tserver when it maps the UUID to an extent.

[dlmarion]

@keith-turner - I modified the TServer and Coordinator pass the Extent to the TServer on compaction completion. I left two CBUG comments in CompactionManager as a reminder to complete the wiring up of the sanity checks.