* Have `linkerd endpoints` use `Destination.Get`
Fixes#2885
We're refactoring `linkerd endpoints` so it hits
directly the `Destination.Get` endpoint, instead of relying on the
Discovery service.
For that, I've created a new `client.go` for Destination and added it to
the `APIClient` interface.
I've also added a `destinationClient` struct that mimics `tapClient`,
and whose common logic has been moved into `stream_client.go`.
Analogously, I added a `destinationServer` struct that mimics
`tapServer`.
Signed-off-by: Alejandro Pedraza <alejandro@buoyant.io>
Alejandro Pedraza