Cleanup monitor code related to getting compactions

[keith-turner]

The monitor was making a thrift call to tservers to get active compactions. This code in the monitor was commented out in #3604. Need to look at what the monitor was doing with this informaiton and clean up the monitor.

[keith-turner]

This may be related to #3594, maybe it makes sense to do both in the same PR. Not sure.

[keith-turner]

The changes in #3921 removed unused code related to getting major compaction information from the tablet servers. Going forward need to decide what to about display per table compaction information in the monitor. Could remove this column from the monitor OR gather the information in a different way. The information on running and queued compactions could still be gathered and displayed, it would just need to be done differently (can not get it from tservers anymore, can get it from monitor and metadata table now).

[dlmarion]

The CompactionCoordinator is running in the Manager, so that information could be returned as part of the ManagerMonitorInfo