Oracle: DRCP and persistent connections support

Adds $wgDBOracleDRCP, when set to true, enables persistent connection
with DRCP on Oracle.

Requires Oracle DB 11gR1 or above. More informations about DRCP can be
found at:
http://www.oracle-base.com/articles/11g/database-resident-connection-pool-11gr1.php

Change-Id: I2be7120a2ebec39d866b79be62ac715bbc3738a9
Merge "Oracle: DRCP and persistent connections support"