Warp: single execution should observe only first request

Default Warp execution mode in Alpha1 was enhancing just first request in Warp execution cycle.

In Alpha2 with introduction of WarpGroups, this behavior was changed, so user needs to define observer for almost each request for complex pages which loads multiple requests, otherwise Warp fails with message that there are too much requests.

Default behavior in single execution scenario (which means without using request groups) is that only first request matching criteria is enhanced with Warp.

This will also preserve backward compatibility.