This commit extends our healtcheck with an optional leader check. This
is to ensure that given network partition or other cluster wide failure
we act as soon as possible to avoid a split-brain situation where a new
leader is elected but we still hold onto our etcd client.
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.
