Switch query service maxlag to be median +1

Median is okay, but there are some situations where this sucks.
(See linked ticket to do with deadlocks)
This is exacerbated by the fact traffic is not evenly split between
eqiad and codfw.

Bug: T242453
Bug: T238751
Bug: T221774
Change-Id: I1f6f7d3f732254a72e59a3d4ac6871364a082991