improve meek behavior when target server is down

As part of our testing of Tor Launcher / Moat functionality, Mark and I ran our own meek-server but intentionally stopped the BridgeDB/moat server to which it was supposed to talk. This caused a 5 minute hang within Tor Launcher before an error was generated.

On the meek-client side, we see a series of messages like these: status code was 500, not 200; trying again after 30 seconds (9)

On the meek-server side, we see these messages: dial tcp 192.168.1.xx:6790: getsockopt: connection refused

Because this is part of the Moat client implementation inside Tor Launcher, if BridgeDB is down a real person will be waiting a long time without receiving any feedback. It does not look like the retry interval or count is configurable within meek-client.

Do you have any suggestions for minimizing or eliminating the 5 minutes? We could implement a different maximum timeout inside Tor Launcher, although knowing that the underlying error is "connection refused" vs. "the network is just really slow" would make things more robust.