[core] Some plasma client cleaning

[dayshah]

Why are these changes needed?

Cleaning out plasma and the plasma client and its neighbors.

The plasma client had a pimpl implementation, even though we didn't really need anything that would come with pimpl out of plasma. So just killing the separate impl class and just having the plasma client and its interface. One note about this is that it needs shared_from_this and the old plasma client would always contain a shared ptr to the impl, so had to refactor the raylet to use a shared ptr to the plasma client so we could keep using the shared_from_this.

Other cleanup:

• a lot of the ipc functions always returned status::ok so changed to void
• some extra reserving of vectors and moving
• unnecessary consts in pull manager that would prevent moves
  etc.