TheSchwartz is a reliable job queue system. Your application can put jobs
into the system, and your worker processes can pull jobs from the queue
atomically to perform. Failed jobs can be left in the queue to retry later.

_Abilities_ specify what jobs a worker process can perform. Abilities are
the names of 'TheSchwartz::Worker' subclasses, as in the synopsis: the
'MyWorker' class name is used to specify that the worker script can perform
the job. When using the 'TheSchwartz' client's 'work' functions, the
class-ability duality is used to automatically dispatch to the proper class
to do the actual work.

TheSchwartz clients will also prefer to do jobs for unused abilities before
reusing a particular ability, to avoid exhausting the supply of one kind of
job while jobs of other types stack up.