Configurable log sampling via $wgDebugLogGroups

Configurable log sampling via $wgDebugLogGroups

Make it possible to configure a log-group-specific sampling factor by allowing
$wgDebugLogGroups values to be associative arrays with 'destination' & 'sample'
keys.

The immediate use-case I have in mind is the memcached log, which is too
verbose to enable in production. We could sample on the receiving end, but
doing so would not help MediaWiki, which would still be on the hook for
processing and dispatching every single record.

Idea was vetted by Dr. Manhattan:

<TimStarling> I think you could have each element in $wgDebugLogGroups be an
  associative array
<TimStarling> e.g. array( 'sample' => 1000, 'target' => "$host:$port")
<TimStarling> that would allow for more features to be added to it in
  future

Change-Id: Ib0ece5d5d9c2aadef13b77ac38995b0e47ac086f