Enable SQLite stat3 for better query optimization

Summary:
SQLITE_ENABLE_STAT3 compile option is added for better query
optimization with "ANALYZE" command.

Test Plan: will probably conflict with D391. I was unable to find a proper test for this, but will find or make something for it (we need tests for this upstream anyway).

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D392