[JobQueue] Added JobQueueFederated class for partitioned queues.

* This lets queues be horizontally partitioned onto
  different servers, with weights assigned to each.
  The queue classes used by the different partitions
  can be hetereogenous or homogeneous.
* How partitioning is done is setup similar to LBFactory,
  where wikis belong to sections and sections have config.

Change-Id: I44d59b67cf417dca28a3e9b25371dac5a7ffcb47