WireMockServerRunner in proxy mode now uses existing recordings

When the server runner is started in proxy mode with recordings enabled, it does not consider any existing recordings already specified in the mappings folder. It will end up recording latest responses for urls that may already be recorded. This creates multiple recordings and may have unintended effects. The solution in this commit is to de-prioritize the proxy-based stub mapping so that existing recordings (usually with default priority) are looked up first.

Corresponding acceptance test added.