With this commit we create a new function that returns system wide
unique ports by using a single file to keep track of previously used
ports. We'll want to use this everywhere whenever we need to listen on a
new, random port during unit or integration tests.
Because we now have a unique source, we don't need to apply the port
offset that was used for the different tranches of parallel running
integration tests before.
Using the default, global ServeMux prevents the same process from
calling `lntest.NewNetworkHarness` multiple times, because we get a
panic when registering HTTP routes.
Instead, we use the ServeMux beloning to the fee service struct.
