rgw/dbstore: DB backend for RGW

rgw/dbstore: DB backend for RGW

As part of Zipper, adding support for DB backend (using SQLite) for Stackable RGW.

The base class methods implemented are generic which could be extended to any backend database, not just SQLite.

More details on design/implementation can be found at -
https://docs.google.com/document/d/1xCoHT5DCujqbe1pnEfYpSaSqiBcW8z87CgQCC5LivOo/edit#

Current status:

[Done]
- User related APIs
- Bucket APIs
- Testcases (using Gtest) to test the DBStore APIs

[Not handled in the first pass]
- Quota
- Usage Stats
- Swift Users
- Multiple Zones/Zonegroups

[WIP]
- Object APIs

Signed-off-by: Soumya Koduri <skoduri@redhat.com>