Have ExternalServer dynamically find available ports

[ScottDugas]

Developers have run into a fair number of issues with servers having different paths (e.g. if they have two copies of record layer), and thus not being cleaned up by the store. Instead, it now dynamically picks the ports (within a range), so if you have some leftover processes sitting around, new processes will find different ports.

This still leaves the code that kills processes around, so that if you keep killing tests, you don't end up with hundreds of servers running.