core: Wait for backoff timer on address update in pick_first

The backoff timer is only used when serializeRetries=true, and that
exists to match the old/current pick_first's behavior as closely as
possible. InternalSubchannel.updateAddresses() would take no action when
in TRANSIENT_FAILURE; it would update the addresses and just wait for
the backoff timer to expire.

Note that this only impacts serializeRetries=true; in the other cases we
do want to start trying to the new addresses immediately, because the
backoff timers are in the subchannels.

Author: ejona86

core/src/main/java/io/grpc/internal/PickFirstLeafLoadBalancer.java

@@ -496,7 +496,7 @@ private void shutdownRemaining(SubchannelData activeSubchannelData) {
    */
   @Override
   public void requestConnection() {
-    if (!addressIndex.isValid() || rawConnectivityState == SHUTDOWN) {
+    if (!addressIndex.isValid() || rawConnectivityState == SHUTDOWN || reconnectTask != null) {
       return;
     }
 

core/src/test/java/io/grpc/internal/PickFirstLeafLoadBalancerTest.java

@@ -1446,6 +1446,11 @@ public void updateAddresses_disjoint_transient_failure() {
     loadBalancer.acceptResolvedAddresses(
         ResolvedAddresses.newBuilder().setAddresses(newServers).setAttributes(affinity).build());
 
+    if (serializeRetries) {
+      inOrder.verify(mockSubchannel3, never()).start(stateListenerCaptor.capture());
+      fakeClock.forwardTime(1, TimeUnit.SECONDS);
+    }
+
     // subchannel 3 still attempts a connection even though we stay in transient failure
     assertEquals(TRANSIENT_FAILURE, loadBalancer.getConcludedConnectivityState());
     inOrder.verify(mockSubchannel3).start(stateListenerCaptor.capture());