Provide a database module

Summary:
The database module allows to create a r2d2 pool, with PostgreSQL
Diesel connections.

The module has been prepared refactoring the code to initialize
a database pool or to test if a database connection works.

Some of the code is extracted from the Rocket manual,
see https://rocket.rs/guide/state/#databases

Test Plan:
Examples provided for documentation are extracted
from API production code

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1662