Lower CDN cache TTL when slave lag is high

Lower CDN cache TTL when slave lag is high

* $wgCdnMaxageLagged controls exactly what that TTL is
  and the usual "max lag" settings determine what "high"
  is for lag (which already makes the site read-only).
* This helps avoids stale content getting stuck in CDN
  for a month just because a slave was lagged for a minute.
  Of course race conditions with normal slave lag and WAN
  cache relay purges can still lead to this problem, though
  the scope of it is reduced.

Bug: T113204
Change-Id: I7ff0a8d88665f4e557566e7b412e75edee2627fe