unix: add send_queue_size to uv_udp_t

This functionality is present in stream and uv_udp_t has a queue
as well so it makes sense for udp to have a send_write_size.
win: add send_queue_size to uv_udp_t
test: add udp.send_queue_size tests
unix: add send_queue_count to uv_udp_t
win: add send_queue_count to uv_udp_t
unix: add uv_udp_try_send
win: add uv_udp_try_send