SendDatagram might block, but there's no context

[marten-seemann]

SendDatagram blocks, once a (fairly large) number of datagrams is queued. This shouldn't happen too often in practice, but it might when the congestion window is suddenly reduced. This would make the sender stuck on SendDatagram.

There should be some way to cancel calls to SendDatagram, short of killing the entire connection.

Related: #4471.