Instead of creating a go routine to process each incoming datagram from the tunnel, a single consumer (the demuxer) will
process each of the datagrams in serial.
Registration datagrams will still be spun out into separate go routines since they are responsible for managing the
lifetime of the session once started via the `Serve` method.
UDP payload datagrams will be handled in separate channels to allow for parallel writing inside of the scope of a
session via a new write loop. This channel will have a small buffer to help unblock the demuxer from dequeueing other
datagrams.
ICMP datagrams will be funneled into a single channel across all possible origins with a single consumer to write to
their respective destinations.
Each of these changes is to prevent datagram reordering from occurring when dequeuing from the tunnel connection. By
establishing a single demuxer that serializes the writes per session, each session will be able to write sequentially,
but in parallel to their respective origins.
Closes TUN-9882
New session manager leverages similar functionality that was previously
provided with datagram v2, with the distinct difference that the sessions
are registered via QUIC Datagrams and unregistered via timeouts only; the
sessions will no longer attempt to unregister sessions remotely with the
edge service.
The Session Manager is shared across all QUIC connections that cloudflared
uses to connect to the edge (typically 4). This will help cloudflared be
able to monitor all sessions across the connections and help correlate
in the future if sessions migrate across connections.
The UDP payload size is still limited to 1280 bytes across all OS's. Any
UDP packet that provides a payload size of greater than 1280 will cause
cloudflared to report (as it currently does) a log error and drop the packet.
Closes TUN-8667
Devin Carr