Zbatery 4.0.0 - another Rainbows! resync
This gets most of the improvements Rainbows! 4.0.0 got:
* client_max_header_size directive is added to limit per-client
memory usage in headers.
* An experimental StreamResponseEpoll concurrency option
* minor bugfixes, minor stack depth reduction
Since Zbatery doesn't fork workers, the ability of Unicorn 4.x
to scale to a large amount of worker processes doesn't matter
to us.