Previously, nghttpx will use only one single thread inside the worker
process if --workers=1 (this is default).  If --workers=N, N > 1, we
use additional threads for accepting connections, or API request
processing, etc.

With this commit, we use the same processing model for N > 1 even if N
== 1.  To restore the original single thread execution mode,
--single-worker option is added.  If threading is disabled
--single-worker is always true.

This commit adds configuration revision, which is considered opaque
string, and changes after reloading configuration with SIGHUP.  This
revision is returned as a response to configrevision API endpoint.
This allows external application to know whether nghttpx has finished
reloading new configuration or not.  Note that this revision does not
change on backendconfig API calls.

nghttpx: Redirect to HTTPS URI with redirect-if-no-tls parameter in backend option

This commit removes frontend-tls parameter, and adds
redirect-if-not-tls parameter parameter to --backend option.  nghttpx
now responds to the request with 308 status code to redirect the
request to https URI if frontend connection is not TLS encrypted, and
redirect-if-no-tls parameter is used in --backend option.  The port
number in Location header field is 443 by default (thus omitted), but
it can be configurable using --redirect-https-port option.

Add TLSv1.3 support

If OpenSSL supports TLSv1.3, enable it by default for all applications
under src.  BoringSSL can work at the moment although it does not
unlock all the features nghttpx offers.  OpenSSL's TLSv1.3 support is
still WIP at the time of writing.

nghttp2_session: fix The 'then' statement is equivalent to the subseq…