Property tests updates for ssh client server tests

Thoose proposed updates makes the QuickCheck ssh stateful tests better by:

1) removing side effects in the generators (Thanks to Thomas Arts @ QuviQ)
2) adding timeouts (Thanks to John & Tobias @ QuviQ)
3) cleaner address and port selection

The items 1) and 2) together aims towards making the tests run-able at the free www.quickcheck-ci.com service.