Investigate half duplex

[cbeuw]

@notsure2 mentioned SoftEther's half duplex mode #283 (comment)

https://www.softether.org/4-docs/1-manual/2._SoftEther_VPN_Essential_Architecture/2.1_VPN_Communication_Protocol#Using_in_Half_Duplex_Mode

The half duplex mode is a function whereby, if VPN communications are conducted by establishing 2 or more TCP/IP connections, concerning various TCP/IP connections between VPN connection source and SoftEther VPN Server, approximately half of the TCP/IP connections are dedicated to the transmission direction and the other half are dedicated to receiving. If this function is enabled, transmission direction of data flowing through respective TCP/IP connections established as part of SoftEther VPN protocol is limited to either from VPN server to client (download) or from client to VPN server (upload). If all TCP/IP connections are lumped together, simultaneous communication in both directions is possible (full duplex), but each respective TCP/IP connection can only handle data transmission in one direction, so it is referred to as the half duplex mode.

This may be useful to defeat size/directionality statistical detection, forcing censors to combine data stream across multiple connections which would be more expensive

Dedicating a TCP connection to be full up or downstream is probably not a good idea, as most TCP traffic are bidirectional, but we can do a x% up 1-x% down approach