specifiy parameter 'queue' to define AJAX queue size

The 2.0 spec specifies an unlimited ajax queue, though for most usages a queue
size of 1 is appropriate.

This param makes the queue size configurable. Follow up requests would replace
prior request if the queue is full.

On jsr-314-open it was discussed whether queue size is a general config param,
but this doesn't hit the nail:

The desired queue size can differ with each use case, so it is meant to define
the number of queue elements one use case tolerates before throwing one out, so
f:ajax/jsf.ajax.request are the right place to put it.
#### Environment
Operating System: All
Platform: All
#### Affected Versions
[2.0]

Start with the f:ajax and jsf.ajax.request entry points named in the issue, then trace how the AJAX request queue is configured and enforced. Check the existing JSF AJAX tests before defining coverage for a per-use-case queue size and replacement of older requests when full.

Written by the indexing model from the issue text.