allow using the method name login() for the login action in UserSessionRule

UserSession is now an interface
the old UserSession has been split into ServerUserSession (implements the lazy loading of credentials directly from DAOs) and ThreadLocalUserSession (implements the thread local management)
add UserSessionRule for UserSession management in unit test with support for ServerTester
DoPrivileged now restores the user session which was in ThreadLocal before it ran
move MockUserSession to server.tester package

using an enum allows testing all possible state of the Platform class based on a single value instead of combining multiple sequential method call which can return an inconsistent result of state changes in between method calls

SONAR-6517 fix regression - server should stop if plugin is not compatible with SQ version (see SONAR-3516)