CRM_Queue_Queue - Add 'getStatistic($name)'. Deprecate 'numberOfItems()'

CRM_Queue_Queue - Add 'getStatistic($name)'. Deprecate 'numberOfItems()'

Should `numberOfItems()` tell you the _total_ number of items -- or the
number of _ready_ items?  Trick question.  They can tell you different
things:

* When implementing a task-worker (which should start/stop based
  on available tasks), you'd use _ready_ items.
* When implementing `existsQueue()` (which tells if you the queue
  has anything at all), you'd use _total_ items.

This trick-question is why different implementations have computed
`numberOfItems()` in different ways.

This patch resolves the trick-question by deprecating `numberOfItems()`
and replacing it with `getStatistic($name)`.