With array, we can control the order of header field directly.

Don't close channel to avoid potential write-after-close.
Use time.After instead of time.NewTimer

The integration tests reside in integration-tests directory.  To run
integration tests, cd integration-tests, then run "go test".  Tests
depends on https://github.com/bradfitz/http2.

g++-4.7 lacks thread_local, which can be workaround by
--disable-threads.  What left remaining is std::map::emplace, which is
what this change deals with.  First check availability of
std::map::emplace, if there is none, use std::map::insert.

This change makes sure that GOAWAY which terminates session is
immediately sent without blocked by other frames.
NGHTTP2_ERR_SESSION_CLOSING library error code was added to indicate
this situation to callback.

So that you can specify --read-rate=1M --read-burst=4M