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.

Added macros which log messages from the following components are
prefixed with their component name + object pointer address:

ListenHandler: LISTEN
ThreadEventReceiver: THREAD_RECV
Upstream: UPSTREAM
Downstream: DOWNSTREAM
DownstreamConnection: DCONN
SpdySession: DSPDY

This avoids the need to provide the password for your
private key interactively.

It can be used via --private-key-passwd-file or private-key-passwd-file
in the given config file. The first line in the file
(without \n) will be treated as the passwd. There isn't
any validation and all lines after the first one (if any)
are ignored.

The security model behind this is a bit simplistic so I
am open to better ideas. Basically your password file
should be root:root (700) and you *should* drop root
and run as an unprivileged user.

If the file exists and a line can be read then a callback
will be set for the SSL ctxt and it'll feed the passwd
when the private key is read (if password is needed).

If the file exists with the wrong permisions it'll be
logged and ignored.