[phabricator] Add mysql slave and read-only database connections

Summary:
Add ability to define mysql slaves and then use that connection on 'r'
connection modes. 'w' connections go to the master server.

Test Plan:

• php -l and checkModule
  • worked in my devbox

Reviewed By: jungejason
Reviewers: dpepper, tuomaspelkonen, jungejason
CC: jungejason, aran
Revert Plan:
sure

Differential Revision: 175