Update grpclb proto and move grpclb into package grpc

[menghanl]

To support loadBalancingPolicy field in service config, gRPC needs to import grpclb as a dependency. This causes a circular dependency: grpc->grpclb->grpc.

To resolve the circular dependency, this change moves the implementation of grpclb into grpc package.

[tamird]

@dfawley it looks like grpclb/grpc_lb_v1/grpclb.pb.go was edited by hand here, which is cause make proto to produce import cycles.

[menghanl]

Yes, grpclb is kind of special and is implemented in grpc package, so we need to edit this file to avoid import cycles at the first place.
Please ignoregrpclb.proto when regenerating pb.go files.