rpcclient: safe read and write to batch

[ynewmann]

This PR tries to make batchList concurrent-safe.

The Send() could panic at the line 1730 if c.removeRequest returned nil. It could return nil if we try to call Send() concurrently.

btcd/rpcclient/infrastructure.go

Lines 1708 to 1730 in 684d64a

	request := c.removeRequest(id)
	// If there's an error, we log it and continue to the next
	// request.
	fullResult, err := json.Marshal(resp.Result)
	if err != nil {
	log.Errorf("Unable to marshal result: %v for req=%v",
	err, request.id)
	continue
	}
	// If there's a response error, we send it back the request.
	var requestError error
	if resp.Error != nil {
	requestError = resp.Error
	}
	result := Response{
	result: fullResult,
	err: requestError,
	}
	request.responseChan <- &result

After the quick look I've noticed that there are places where the batchList is update unsafely. So, the simplest fix is just to add locks, but I'm afraid if it could break something else, because I'm not very familiar with the library.