Notify parent ClientConn to re-resolve in grpclb (#1699)

The parent ClientConn should re-resolve when grpclb loses connection to the
remote balancer.

When the ClientConn inside grpclb gets a TransientFailure, it calls
lbManualResolver.ResolveNow(), which calls parent ClientConn's ResolveNow, and
eventually results in re-resolve happening in parent ClientConn's resolver (DNS
for example).

This PR adds a method to balancer.ClientConn interface, so balancer can tell
parent ClientConn to re-resolve.

balancer/balancer.go

@@ -109,6 +109,9 @@ type ClientConn interface {
 	// on the new picker to pick new SubConn.
 	UpdateBalancerState(s connectivity.State, p Picker)
 
+	// ResolveNow is called by balancer to notify gRPC to do a name resolving.
+	ResolveNow(resolver.ResolveNowOption)
+
 	// Target returns the dial target for this ClientConn.
 	Target() string
 }