Provide ajax function to get the number of requests on the queue

Customer request: Provide a jsf.ajax function to query the number of requests
that are currently waiting on the queue.

Easy to implement, and could conceivably be useful to a small class of users.
#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.0]

Start by locating the jsf.ajax queue implementation and its existing tests or public API entry points. Clarify how queued requests are counted, including requests in progress, then define the query's expected result and add coverage showing that it reports the current queue size.

Written by the indexing model from the issue text.