Rainbows! 0.91.0 - Unicorn resync

Unicorn 0.97.0 has a bunch of internal cleanups and small fixes
and this is mainly to resync with those changes.

keepalive_timeout now defaults to 5 seconds (from 2 seconds
previous).  This should help out clients on slower connections.

Some small fixes and cleanups:

* Rainbows::Fiber::IO objects may leak if a rare app uses them
  explicitly with FiberSpawn/FiberPool-only (not RevFiberSpawn)

* quiet down ENOTCONN handling, there's nothing we can do about
  this error so we won't fill our logs with it.