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.
This commit adds a new build tag `integration` and removes the old tag
`rpctest` for clarity. Multiple unnecessary usages of `build !rpctest`
is also removed.
