Re-resolve target when one connection becomes TransientFailure (#1679)

This allows ClientConn to get more up-to-date addresses from resolver.
ClientConn compares new addresses with the cached ones. So if resolver returns the same set of addresses, ClientConn will not notify balancer about it.

Also moved the initialization of resolver and balancer to avoid race. Balancer will only be started when ClientConn gets resolved addresses from balancer.

resolver_conn_wrapper.go

@@ -87,10 +87,13 @@ func newCCResolverWrapper(cc *ClientConn) (*ccResolverWrapper, error) {
 	if err != nil {
 		return nil, err
 	}
-	go ccr.watcher()
 	return ccr, nil
 }
 
+func (ccr *ccResolverWrapper) start() {
+	go ccr.watcher()
+}
+
 // watcher processes address updates and service config updates sequencially.
 // Otherwise, we need to resolve possible races between address and service
 // config (e.g. they specify different balancer types).
@@ -125,6 +128,10 @@ func (ccr *ccResolverWrapper) watcher() {
 	}
 }
 
+func (ccr *ccResolverWrapper) resolveNow(o resolver.ResolveNowOption) {
+	ccr.resolver.ResolveNow(o)
+}
+
 func (ccr *ccResolverWrapper) close() {
 	ccr.resolver.Close()
 	close(ccr.done)