Get rid of scalapb

[fit51]

We don't really need to generate etcd protoc sources.

We could use io.etcd.jetcd v 0.0.3 with it's builders and KV client. (v0.0.1 was not suitable for us in the past)
Watch implementation however is quite primitive there, so we will stay on our own implementation.

Moving to io.etcd.jetcd would force us to use java CompletableFuture instead of scala Future.
This may add cancellation functionality (check needed).

I suppose it's a good idea to create a proper client in separate project monix-etcd.