Thold cannot alert on CDEF/calculated data source values

[somethingwithproof]

Summary

Thresholds configured against data sources that use CDEF (calculated) values do not trigger alerts. The threshold check does not evaluate the CDEF-transformed value, so breaches go undetected.

Fix

Implementation in #798.

[somethingwithproof]

Provenance: a07df084 (2019-09-11, "Thresholds are not applying CDEF's correctly").

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