The GRPCLB load-balancer.

This LoadBalancer does round-robin on a address list received from a
separate "load-balancer service", via the protocol defined in
load_balancer.proto. Everything is put under a subproject `grpc-grpclb`,
because it has dependency to protobuf.

updateRetainedTransports() now accepts EquivalentAddressGroups.  The
LoadBalancer merges the LB and normal server address groups when calling
it.
diff --git a/settings.gradle b/settings.gradle
index 27d128e..f235a33 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -6,6 +6,7 @@
 include ":grpc-protobuf"
 include ":grpc-protobuf-nano"
 include ":grpc-netty"
+include ":grpc-grpclb"
 include ":grpc-testing"
 include ":grpc-interop-testing"
 include ":grpc-all"
@@ -19,6 +20,7 @@
 project(':grpc-protobuf').projectDir = "$rootDir/protobuf" as File
 project(':grpc-protobuf-nano').projectDir = "$rootDir/protobuf-nano" as File
 project(':grpc-netty').projectDir = "$rootDir/netty" as File
+project(':grpc-grpclb').projectDir = "$rootDir/grpclb" as File
 project(':grpc-testing').projectDir = "$rootDir/testing" as File
 project(':grpc-interop-testing').projectDir = "$rootDir/interop-testing" as File
 project(':grpc-all').projectDir = "$rootDir/all" as File