Add retry loop when trying to establish db connection, log retries

Summary:
We retried if a db connection was lost when executing a query, but not when
establishing a connection. I've seen a lot of failures establishing connections
in our install (they go away when retrying), so this diff retries when
establishing connections, and logs when we retry.

Test Plan:
- Loaded phabricator in a sandbox
- Temporarily added a check in the try block to throw if there were still
  retries (to test logging, retry logic)

Reviewers: epriestley, blair

Reviewed By: epriestley

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1460

conf/default.conf.php

@@ -117,6 +117,9 @@ return array(
   // (e.g., db.example.com:1234).
   'mysql.host' => 'localhost',
 
+  // The number of times to try reconnecting to the MySQL database
+  'mysql.connection-retries' => 3,
+
 
 // -- Email ----------------------------------------------------------------- //