Don't wait for the whole server list before accepting connections

Blocking until all servers have been checked is safe, but significantly
increases startup times.

OTOH, we shouldn't accept connections unless we have at least one live
server.

So, a better approach may be to add the ability for `serversInfo.refresh()`
to write to a channel after a live server has been found, and block on
that channel in the main thread before accepting client connections.

dnscrypt-proxy/proxy.go

@@ -215,6 +215,7 @@ func (proxy *Proxy) StartProxy() {
 	for _, registeredServer := range proxy.registeredServers {
 		proxy.serversInfo.registerServer(registeredServer.name, registeredServer.stamp)
 	}
+	proxy.startAcceptingClients()
 	liveServers, err := proxy.serversInfo.refresh(proxy)
 	if liveServers > 0 {
 		proxy.certIgnoreTimestamp = false
@@ -233,7 +234,6 @@ func (proxy *Proxy) StartProxy() {
 		dlog.Error(err)
 		dlog.Notice("dnscrypt-proxy is waiting for at least one server to be reachable")
 	}
-	proxy.startAcceptingClients()
 	go func() {
 		for {
 			clocksmith.Sleep(PrefetchSources(proxy.xTransport, proxy.sources))