With this commit, SSL_TLSEXT_ERR_NOACK is returned from
servername_callback, which removes server_name extension from
ServerHello.  CertLookupTree is now used even if the number of server
certificate is one.  It is better to exercise it regularly.

nghttpx: Forward multiple via, xff, and xfp header fields

Previously, for Via, X-Forwarded-For, and X-Forwarded-Proto header
field, nghttpx only forwarded the last header field of each.  With
this commit, nghttpx forwards all of them if it is configured to do
so.

nghttpx: SNI based backend server selection

nghttpx: Add $tls_sni access log variable

nghttpx: Rename ssl_* log variables as tls_*

The exiting ssl_* log variables still work for backward compatibility.

nghttpx: Fix path matching bug

Previously, if path is empty or path does not start with "/", nghttpx
did not try to match with wildcard pattern.  This commit fixes it.

Update nghttp2_session.c

Add missing free call on error in inflight_settings_new().

Support specifying stream priority via session::submit()