Summary:
If we have total number of sorted runs greater than level0_file_num_compaction_trigger, Universal compaction will always issue a compaction
even if the number of sorted runs that are not being compacted is less than level0_file_num_compaction_trigger.

This diff changes this behaviour to relay on the `number of sorted runs not being compacted` instead of `total number of sorted runs`

Test Plan: New unit test

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61533