Remove SO_REUSEPORT from DnsTlsFrontend

It doesn't just let us "share" the port with another server, it also
load-balances connections, making things mysteriously flaky.

There's probably a race between on DnsTlsFrontend quitting and the next
one starting when multiple tests run... but this isn't the right fix.

Change-Id: I0221f2ca9424222b00a283ff5cbfabd0169bd616