option to bind a new port for each process using SO_REUSEPORT(_LB)

[graingert]

the linux kernel supports load balancing connections to a group of processes each using a new socket.bind using the SO_REUSEPORT flag. FreeBSD supports the same feature using SO_REUSEPORT_LB

supervisor should support an option to bind a new port for each process using this flag, as this would help improve performance for some workloads that require fair load balancing, that sharing a single socket between multiple processes does not.

see https://lwn.net/Articles/542629/ and where this issue was originally reported: encode/uvicorn#2467