openssl lacks SSL_CTX_get0_certificates().

To reduce TTFB with large number of incoming connections, we now
intentionally accept one connection at a time, so that it does not
delay the TTFB of the existing connection.  This is significant
especially for TLS connections.

For HTTP/2, read timer starts when there is no downstream, and timer
stops when there is at least one downstream.  For HTTP/1, read timer
starts when request handling finished, and timer stops when request
handling starts.

This commit also fixes the rare issue that connection is not made
properly because of race between nghttpx process and TCP client
connection.

This reverts commit c42296ac.

Now we define NGHTTP2_SSIZE_T which is typedef-ed to the appropriate
type depending on the platform (x86/x86_64).

See GH-616 for details

This commit adds ALPN support to tutorial client/server.  It also adds
a code to check h2 was negotiated, if not, drop connection.

For tutorial server, now it sends connection preface just after TLS
handshake was made without waiting for the client connection preface.

We have to gracefully handle the case where response ends before
request body is fully received.