Removed StartColocationGroup from the pipe API.

[mwhittaker]

Consider a component A that calls weaver.Get on a different component B. Before this PR, the weavelet hosting A would send two messages over the pipe to the envelope.

1. It would send a StartComponent message indicating that the B component should be started by B's colocation group.
2. It would send a StartColocationGroup message indicating that B's colocation group should be started, if it hasn't already.

This PR deletes the unneeded StartColocationGroup message. When a weavelet sends a StartComponent message, the envelope now registers that the component should be started and starts its colocation group, if it hasn't already. This simplifies the pipe API and makes implementing deployers a little bit easier.

I also removed some unneeded fields in the StartComponent proto.