Clean up compaction logging

Summary: Cleaned up compaction logging a little bit. Now file sizes are easier to read. Also, removed the trailing space.

Test Plan:
verified that i'm happy with logging output:

        files_size[#33(seq=101,sz=98KB,0) #31(seq=81,sz=159KB,0) #26(seq=0,sz=637KB,0)]

Reviewers: sdong, haobo, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18549