Fix mac compile

Summary: We should use PRIu64 instead of "%lu" for portability

Test Plan: compiles now

Reviewers: ljin, dhruba

Reviewed By: dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19809
[db stress] Don't drop column families if there's only 1
BG -> GB
Improve Put()'s comment to indicate that the key is overwritten if existing

Summary: As title

Test Plan: Not needed for comment only.

Reviewers: yhchiang, ljin, MarkCallaghan

Reviewed By: MarkCallaghan

Subscribers: xjin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D19887