v2.1.0 of grassroots_dart_udx

Fork of dart_udx (QUIC-inspired reliable, multiplexed, congestion-controlled UDP streams) with raw datagram passthrough and richer send-error reporting, maintained for the Grassroots Networking transp [...] Changelog excerpt: Forked from upstream dart_udx2.0.3 (commit 3f88edbon github.com/danbachar/dart-udx). ### Added - UDXMultiplexer.onRawPacket— callback invoked for datagrams that are not valid UDX packets, instead of silently dropping them. Lets a single UDP socket carry UDX traffic and an application-defined wire format (e.g. signaling, hole-punch coordination) side by side. ### Changed - UDXMultiplexer.sendnow returns SocketException?instead of void. Call[...]