Support bServerSide for datatables (not only views)

[jean]

At the moment, only PlominoView.tojson understands the bServerSide paging parameters (iDisplayStart etc) (see #126).

However datagrid fields on forms may also be paging through large datasets, so their tojson methods also need to understand these parameters.

Since these fields may be dynamically computed per request, per viewer, they need to be cached accordingly. Perhaps the Plomino support for caching in forms can be extended to deal with this. In addition to caching sections of rendered HTML, add a cacheid formula to dynamic fields. By default, this can create a form_field_initialrequest key which tojson can use to find the right cache. The initialrequest id would be computed when the form is rendered, so paging in that form will get that cache.

Field caches can be cleaned whenever the form is rendered or the field is computed or the form is submitted.

Should old caches be dropped? (The last-rendered set of caches for a form will not be cleaned up.) In the case of a dynamic field, yes. In the case of a static field, if we need the RAM. Configure this?

[ebrehault]

I have replace the direct values injection by ajax loading ('sAjaxSource': 'tojson?item=${fieldname}&rendered=true&datatables=true' instead of aaData: [[....]])

so that's a first step