Update tokio-udp to use std-future

[blckngm]

Close #1195.

Changes:

• recv_dgram, send_dgram, RecvDgram and SendDgram are replaced with send, recv,
  send_to and recv_from which return std Futures that borrows the socket, i.e., can be used with async/await. It's roughly the same API that runtime and romio offers. (Close Async/Await interface for UDP #616, Close send_dgram doesn't work with connect on macOS #1126.)

• The poll... functions are updated.

• UdpFramed is removed (for now). It seems that tokio-codec hasn't been updated yet. Besides, I think the framed API is not as useful as it used to be in v0.1:

  • UdpFramed produces/consumes owned buffers, which is the preferred way to deal with buffers when using combinators. But with async/await borrowed buffers can be used just fine.
  • UdpFramed can be split. But it incurs an unnecessary locking overhead. A specialized split should be implemented (like Consider a specialized split fn for TcpStream #174).

• Docs are updated.

• Some basic tests are included. I have fixed and exposed the tokio::test macro and let tokio_udp depend on tokio for testing. Feels quite hacky. Let me know if there are better ways.

[LucioFranco]

Looks good for a first pass

[carllerche]

Looks great 👍

We can polish anything else remaining in follow up PRs.

[carllerche]

Thanks

[carllerche]

Just need @LucioFranco to +1 as well.

[LucioFranco]

LGTM