Allow a limit to be set when deduping

In order to improve dedupe performance it is possible (but not compulsory) to limit by group. This has the effect of restricting the results to contacts either in our out of the group that match that limit.

I propose adding a second limit field that specifies how many contacts to find matches before to give more control over performance, to reduce the effort required to create a group first and to protect over unintentionally killing the site by choosing a large group.

At this stage paging is out of scope and I'm looking at a fairly blunt option that can be used in conjuction with the group limit or not.