Add StatsD metric logging

Add StatsD metric logging

This patch adds a metric data service object to the IContextSource interface,
with full support for StatsD meters, gauges, counters and timing metrics, via
the liuggio/statsd-php-client, which this patch also introduces.

Usage example:

    $stats = $context->getStats();
    $stats->increment( 'resourceloader.cache.hits' );
    $stats->timing( 'resourceloader.cache.rtt', $rtt );

The metrics are flushed to a StatsD server, which may be specified via the
'StatsdServer' configuration key. If no such configuration key exists, the
metrics are discarded.

The StatsD client supplants MediaWiki's StatCounter class. wfIncrStats()
will continue to work, but it will delegate to the StatsD data object.

Change-Id: Ie10db1c154d225971398e189737de7c560bf0f90