This PR splits out grpclb from grpc.  I have made the PR in several commits so you can see more clearly the steps that happened.

There are a few possibly contentious points that I would like to make clear up front:

* grpclb will no longer autoload as a load balancer.  I think this is okay, as service config is not widely (at all?) used, and I believe this is the only way to access it.
* `internal` is used more, as a way of having code shared between packages without exposing types
* ConnectivityStateEvaluator, as used by grpclb, is no longer thread safe.  I believe there is an outer mutex that guards access, but I want to point out this subtle change up here.

All but one tests pass with this, due to another cyclic dependency.  I can fix this, but it is a little more widely scoped (such as exposing grpc.server and grpc.errorDesc in the internal package).   This PR is a nearly-passing sample of that last step to get this working. 

PTAL @menghanl @dfawley