Summary: Implement a time series database that supports DateTieredCompactionStrategy. It wraps a db object and separate SST files in different column families (time windows).

Test Plan: Add `date_tiered_test`.

Reviewers: dhruba, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D61653