Slimmer rewrite of socket_forward_proxy

Key differences:
 1. No QueueState
 2. No sequence number
 3. No generation number

Instead, the guest-side monitors all queues for new connections and the
host-side keeps track of which queues are allocated

design: https://docs.google.com/document/d/1z43c9LGeEEU6G-ojNtEeQP9-ezK890MU3GP6df3byYs

Change-Id: If0396de2ef8080ed78e7afc36ac0d661f99b6d3c
Bug: 80104636
Bug: 110707067
Test: run local while restarting the host-side process and guest-side
process every ~10 seconds. Connect to a guest-side server that only
sends (doesn't receive) information to the host. Connect several
host-side clients to a guest-side echo server. Restart without having
sent any data in either direction or in only one direction.