FTS / moss - add option (CompactionMaxSegments) to compact when max # segments reached

Compaction percentage is calculated (roughly) as (size of new data) / (size of existing data)

during the lifetime of an index, the "size of existing data" grows larger and larger, if I set my compaction ratio is set as 0.5 then the number of segments contributing to the "size of new data" has to grow very tall in order to reach the ration of 0.5. Over time, this excessive height of the segment stack will affect scan performance.