introduce WhenThenCheck to allow to use RestxSpecRule with custom checks
introduce RestxServerRule: a junit rule to start and stop a restx server

based on a split of RestxSpecRule
add getPort on RestxRequest
review RestxMainRouterFactory implementation, fixing concurrency issues

It's now a real factory, and does not implement RestxMainRouter anymore.
 That way it fixes a concurrency issue when using onrequest mode with SimpleWebServer.

 Moreover it's not necessary anymore to set restx.baseUri system property, it's deduced automatically.