Summary:
1. fix possible overflow of the two stats by using uint64_t
2. use a similar source of data to calculate RecordDrop. Previous one is not correct.

Test Plan: See outputs of db_bench settings, and the results look reasonable

Reviewers: MarkCallaghan, ljin, igor

Reviewed By: igor

Subscribers: rven, leveldb, yhchiang, dhruba

Differential Revision: https://reviews.facebook.net/D28155