[JobQueue] Job queue refactoring and generalizing.

[JobQueue] Job queue refactoring and generalizing.

* Added support for different queue types and methods for storing queues.
* Treat each job type as being on its own queue, at least logically.
* Added $wgJobTypeConf to configure queue types for each job type.
* Improved the job DB table so that duplicate job checks actually work
  and are faster. Also improved the method for popping rows of the table.
* Disabled duplicate job removal for everything except refreshLinks.
  The DELETE statements just add DB overhead and are not useful for cheap
  jobs, especially ones with start/end params (which are unlikely to have
  exact duplicates).

Change-Id: I49824c7fa855fea4ddcac5c9901ece8c2c0101d0