This option specifies additional certificate and private key
file. Shrpx will choose certificates based on the hostname indicated
by client using TLS SNI extension. This option can be used multiple
times.

INFO log and its surrounding code are now guarded by
LOG_ENABLED(SEVERITY) macro so that they don't run if log level
threshold is higher. This increases performance because log formatting
is somewhat expensive.

create_error_html() is rewritten without std::stringstream.