Don't use a default port for creating grpcio channels

[shsms]

What's needed?

We currently use a default port of 9090 if incoming URIs don't specify a port. This is confusing, because grpc doesn't have a default port, and we might even inadvertently connect to a different service.

Proposed solution

There should be no default port, and if a URI doesn't specify a port, we should raise an exception immediately after parsing it.