epoll: ensure closing of pipelined clients if required

epoll: ensure closing of pipelined clients if required

on_read normally relies on the close checking in on_readable,
but on_deferred_write_complete may be called from on_writable
instead (bypassing the close check of on_readable).

This affects both Epoll and XEpoll users, but only if they
were sending responses via body#to_path and triggering
on_deferred_write_complete.

While we're at it, favor non-recursive want_more instead of
calling on_readable recursively in ev_write_responses to
prevent stack growth.