Initialize MySQL instance when needed

Summary:
$db was populated at global level.

This change introduces a singleton pattern in the sql_db class
to load it when needed.

If a container is introduced, sql_db could be a facade for the
SQL service in the container.

Ref T1294

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1294

Differential Revision: https://devcentral.nasqueron.org/D1162