In the tower session itest, we make sure to mine blocks on session count
assertion failure. This required because the session count is expected
to change only when 2 things both happen: 1) a closable session is
queued and 2) a new block notification comes through _after_ the session
has been queued. Only then will it be deleted. So we need to do this to
prevent the case where all the block notifications are consumed _before_
the session is queued for deletion. This flake tended to happen in the
windows itest.
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.
