Add two new methods:
`uv_udp_connect()` to connect / disconnect an UDP handle.
`uv_udp_getpeername()` to get the remote peer address of a connected UDP
handle.
Modify `uv_udp_send()` and `uv_udp_try_send()` to accept a `NULL` `addr`
to send messages over an "UDP connection".
Refs: https://github.com/libuv/leps/pull/10
PR-URL: https://github.com/libuv/libuv/pull/1872
Backport-PR-URL: https://github.com/libuv/libuv/pull/2217
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
