Any exceptions that occured at the view or controller level for a
controller using ActionController::Live would cause the server to either
hang with an open socket indefinitely, or immediately crash (depending
on whether the server was launched with rails s or directly). Changed
the behavior of exceptions to act the same as streaming templates for
html requests, and allow for an on_error callback if needed.
responses.
Processing controller actions in a separate thread allows us to work
around the rack api - we can allow the user to set status and headers,
then block until the first bytes are written. As soon as the first
bytes are written, the main thread can return the status, headers, and
(essentially) a queue for the body.
