Add logging context to database logs

* Add optional $context parameter to wfDebug, wfDebugLog, wfLogDBError
  and wfErrorLog that will be passed to MWLogger.
* Add support for PSR-3 style log message parameter interpolation in
  MWLoggerLegacyLogger.
* Add context information to wfLogDBError calls made from DatabaseBase,
  DatabaseMysqlBase and LoadBalancer instances.
* Deprecate wfDebugTimer() which now appears to be unused.

Change-Id: Ic90d593d00a2b0b5b80ed205908cbe624042603c