Proper multiqueue support for tun devices (#382)

This change is for Linux only.

Previously, when running with multiple tun.routines, we would only have one file descriptor. This change instead sets IFF_MULTI_QUEUE and opens a file descriptor for each routine. This allows us to process with multiple threads while preventing out of order packet reception issues.

To attempt to distribute the flows across the queues, we try to write to the tun/UDP queue that corresponds with the one we read from. So if we read a packet from tun queue "2", we will write the outgoing encrypted packet to UDP queue "2". Because of the nature of how multi queue works with flows, a given host tunnel will be sticky to a given routine (so if you try to performance benchmark by only using one tunnel between two hosts, you are only going to be using a max of one thread for each direction).

Because this system works much better when we can correlate flows between the tun and udp routines, we are deprecating the undocumented "tun.routines" and "listen.routines" parameters and introducing a new "routines" parameter that sets the value for both. If you use the old undocumented parameters, the max of the values will be used and a warning logged.

Co-authored-by: Nate Brown <nbrown.us@gmail.com>

examples/config.yml

@@ -86,6 +86,15 @@ listen:
   #read_buffer: 10485760
   #write_buffer: 10485760
 
+# EXPERIMENTAL: This option is currently only supported on linux and may
+# change in future minor releases.
+#
+# Routines is the number of thread pairs to run that consume from the tun and UDP queues.
+# Currently, this defaults to 1 which means we have 1 tun queue reader and 1
+# UDP queue reader. Setting this above one will set IFF_MULTI_QUEUE on the tun
+# device and SO_REUSEPORT on the UDP socket to allow multiple queues.
+#routines: 1
+
 punchy:
   # Continues to punch inbound/outbound at a regular interval to avoid expiration of firewall nat mappings
   punch: true