Compaction now conditionally boosts the size of deletion entries.

Compaction now conditionally boosts the size of deletion entries.

Summary:
Compaction now boosts the size of deletion entries of a file only when
the number of deletion entries is greater than the number of non-deletion
entries in the file.  The motivation here is that in a stable workload,
the number of deletion entries should be roughly equal to the number of
non-deletion entries.  If we compensate the size of deletion entries in a
stable workload, the deletion compensation logic might introduce unwanted
effet which changes the shape of LSM tree.

Test Plan: db_test --gtest_filter="*Deletion*"

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D38703