Make the format of UDP-logged stats configurable

The format of datagrams generated by StatCounter and ProfilerSimpleUDP is not
described by any standard and appears to have been designed for compatibility
with the logging setup of the Wikimedia Foundation, which limits reusability.

This patch adds two configuration variables, $wgUDPProfilerFormatString and
$wgStatsFormatString, that can be used to specify the precise format that
MediaWiki will use. The default values for these variables causes MediaWiki to
generate the same output as before.

The secret evil plan is to change the defaults so that MediaWiki emits metrics
that are compatible with the popular StatsD format (see
<https://github.com/b/statsd_spec>). This would allow us to replace udpprofiler
with a generic StatsD instance instead.

Change-Id: Iaf00811d3f8d6b89c1c68e84fc1f2c93425d5a2d