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