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.

If --no-via option is given, shrpx does not append to Via header
field. If Via header field is received, it is left unaltered.

upstream_response logs more detailed information.

Color severity level if stderr refers to a terminal.