srm: refactor SchedulerFactory

This patch modifies SchedulerFactory so it has a single concern:
creating Scheduler instances.  The current implementation of
SchedulerFactory has two other concerns: creating all necessary
schedulers and maintaining a collection of schedulers. These
additional activities are moved elsewhere.

Creating all necessary schedulers has moved to the spring XML file.

Most of the behaviour associated with multiple schedules has moved
into the new SchedulerContainer class and the process of submitting a
Job to a scheduler is achieved via this new class.

One design goal is that SchedulerContainer should not 'leak' Scheduler
instances.  The getScheduler-like methods has been removed: both from
SchedulerContainer/SchedulerFactory and from SRM class.  Activity that
iterates over all the Schedulers has moved to SchedulerContainer.
Activity that targets a specific scheduler is implemented with a
method that accepts a Class<? extends Job> argument to target a
specific Scheduler.

Note that the current code has the Job classes being responsible for
adding themselves to a scheduler.  To try to keep the patch short,
this mode of operation is maintained; however, Job is supplied with a
Scheduler rather than discovering the scheduler itself.

The handler classes must be able to add a Job to a scheduler.  The
patch achieves this via the SRM object passed to the handler's
constructor.  Future patch may refactor this so the object to which a
Job is submitted is supplied on the constructor.

Another future direction is to make Scheduler an interface that the
current Scheduler class implements.  The SchedulerContainer would also
implement this interface and the similarity between SchedulerContainer
and Scheduler becomes clearer.

Another beneficial side-effect from this patch is that
SchedulerFactory is no longer a singleton.

Target: master
Requires-book: no
Requires-notes: no
Patch: http://rb.dcache.org/r/6321/
Acked-by: Gerd Behrmann