Process.wait2 waits for specific server PID.

Resolves an issues where `Process.wait2` could hang if the process
had already been killed and there was another child process to wait
for. By waiting for a specific PID we can sidestep this problem and
not hang indefinitely. It would probably be good to have a back-off
and eventually `kill 9` the process if it is not dying.

Fixes #13