Only the protocol, port and address of the listener were used to check if
a listener exists. The check should also use the name of the listener to
be sure that each name is unique.
Expanded tests to check that the creation of duplicate listeners is
detected. Did minor improvements to related test code.
The `cluster diff` command is now tested by doing add/alter/remove
operations on servers. As the same methods are used for all objects, they
should detect all sorts of modifications between two Maxscales.
The test tests the internals of the cluster diff generation
algorithms. This should guarantee that the diff calculation and the
resulting synchronization will work as expected without having to test it
on a live cluster.
