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.