fix race condition when accepting streams

[marten-seemann]

What's happening here is the following: In AcceptStream we check if the stream we want to return already exists: https://github.com/lucas-clemente/quic-go/blob/629272c7138e9f2824d5cbf5d629e801339301d7/streams_map_incoming_generic.go#L64-L73
It does not. Before advance to the select statement, the go routine is preempted and GetOrOpenStream is called and the stream is created. Since newStreamChan is an unbuffered channel, GetOrOpenStream doesn't send on that channel:
https://github.com/lucas-clemente/quic-go/blob/629272c7138e9f2824d5cbf5d629e801339301d7/streams_map_incoming_generic.go#L116-L122
Now AcceptStream will never return the stream, as it's now blocking on the select statement.

The solution is simple: make newStreamChan a channel of capacity 1.