Bind a new socket per-worker

We've seen lumpy distributions of requests to workers, which seems to
parallel what some other projects have seen [0][1].

The solution (as best we can tell) is to take advantage of the
SO_REUSEPORT that eventlet's been setting for us since basically
forever [2].

[0] https://lwn.net/Articles/542629/
[1] varnish/hitch#142
[2] eventlet/eventlet@f9a3074a3

Change-Id: I83cdaa2cbd394cbd49609c65bf9c5ed026c55417

Author: tipabu