CompactionStats to support larger value of RecordIn and RecordDrop

Summary: now we use %8d for RecordIn and %10d for RecordDrop, which is far too small for some use cases. Extend both of them to %12d.

Test Plan: run one test in db_test and see the LOG file.

Reviewers: igor, MarkCallaghan, ljin

Reviewed By: ljin

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D28041