As per discussion in meeting: don't reject entire batch of wifis on one bad key.

The old logic grew as a shortcut to detecting when we were receiving old
style SHA1s from fennec. That never happens anymore and in any case it
does not hurt too much to test each wifi in a batch for validity.
Tighten range on asu and ta.
Inhibit mapstat update if we get no data.
Add testcase for normalization of cell measures.