I've had a couple of instances during stress testing now where
Conn::ready would overflow its stack due to recursing on itself. This
moves subsequent calls to ready() into a loop outside the function.
We observed an issue where connection were not ever entering the
keep-alive state due to a bug with `State::update`. The issue is
resolved with resetting the write state to KeepAlive when it arrives as
KeepAlive. Otherwise, it would be marked incorrectly as Closed.
The `trace!` lines in here are useful for debugging keep-alive issues so
I've left them in.
All of the move semantics remain the same for http::Conn while
the self consumption and move semantics only require a pointer copy
now rather than copying larger amounts of data. This greatly improves
the performance of hyper, by my measurements about 125% faster when
benchmarking using wrk.
