Segregate contributions and lopager traffic on s[234567]

This has been done for ages on s1 to keep expensive IndexPager
queries that hit a lot of cold data from churning up slave buffer
pools. Time has come to do the same for s2 (several large wikis)
and s4 (commons). Principle of least astonishment says the other
shards LB might as well be consistent.

Change-Id: I7517b196ed267c1526cd244ddbd3cfd815deafbc