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

When same SSL_CTX is used by multiple thread simultaneously we have to
setup some number of mutex locks for it.  We could not check how this
locking affects scalability since we have 4 cores at best in our
development machine.  Good side of sharing SSL_CTX across threads is
we can share session ID pool.

If --tls-ctx-per-worker is enabled, SSL_CTX is created per thread
basis and we can eliminate mutex locks.  The downside is session ID is
no longer shared, which means if session ID generated by one thread
cannot be acceptable by another thread.  But we have now session
ticket enabled and its keys are shared by all threads.

With androideabi-4.9, android build now supports threading.

This also fixes the bug that nghttpx's acceptor fd is blocking if
SOCK_NONBLOCK is undefined.

Thanks to Pascal

Connection failure is handled by connected() and other errors lead to
just failure, no retry is needed.