Fix hang when Websocket Read is interrupted

Updated to catch the quit signal and break out of the loop. Tested that ^C shutsdown `btcwallet` as expected, with the same env in which it was hanging earlier.
And here I would say:

```
// A for-select with a read of the quit channel is used instead of a for-range
// to provide clean shutdown.  This is necessary due to WebsocketClientRead
// (which sends to the allRequests chan) not closing allRequests during shutdown
// if the remote websocket client is still connected.
for {
...
```