core, grpclb: change policy selection strategy for Grpclb policy (take two: move logic of querying SRV into Grpclb's own resolver) (#6723)

Eliminated the code path of resolving Grpclb balancer addresses in grpc-core and moved it into GrpclbNameResolver, which is a subclass of DnsNameResolver. Main changes:

- Slightly changed ResourceResolver and its JNDI implementation. ResourceResolver#resolveSrv(String) returns a list of SrvRecord so that it only parse SRV records and does nothing more. It's gRPC's name resolver's logic to use information parsed from SRV records.

- Created a GrpclbNameResolver class that extends DnsNameResolver. Logic of using information from SRV records to set balancer addresses as ResolutionResult attributes is implemented in GrpclbNameResolver only.

- Refactored DnsNameResolver, mainly the resolveAll(...) method. Logics for resolving backend addresses and service config are modularized into resolveAddresses() and resolveServiceConfig() methods respectively. They are shared implementation for subclasses (i.e., GrpclbNameResolver).

Author: voidzcy

api/src/test/java/io/grpc/NameResolverRegistryTest.java

@@ -21,7 +21,6 @@
 import static org.mockito.Mockito.mock;
 
 import io.grpc.NameResolver.ServiceConfigParser;
-import io.grpc.internal.BaseDnsNameResolverProvider;
 import io.grpc.internal.DnsNameResolverProvider;
 import java.lang.Thread.UncaughtExceptionHandler;
 import java.net.URI;
@@ -155,10 +154,11 @@ public void newNameResolver_noProvider() {
   public void baseProviders() {
     List<NameResolverProvider> providers = NameResolverRegistry.getDefaultRegistry().providers();
     assertThat(providers).hasSize(2);
-    // 2 name resolvers from grpclb and core
-    for (NameResolverProvider provider : providers) {
-      assertThat(provider).isInstanceOf(BaseDnsNameResolverProvider.class);
-    }
+    // 2 name resolvers from grpclb and core, ordered with decreasing priorities.
+    assertThat(providers.get(0).getClass().getName())
+        .isEqualTo("io.grpc.grpclb.SecretGrpclbNameResolverProvider$Provider");
+    assertThat(providers.get(1).getClass().getName())
+        .isEqualTo("io.grpc.internal.DnsNameResolverProvider");
     assertThat(NameResolverRegistry.getDefaultRegistry().asFactory().getDefaultScheme())
         .isEqualTo("dns");
   }