validation: Reduce O(N**2) edge comparison to O(N)

[arjclark]

The current edge comparison routine under validation used to check for cyclic dependencies is inefficient. For suites with very large numbers of inter task dependencies e.g. LARGE_FAM1:succeed-all => LARGE_FAM2 the current calculation means the suite is effectively un-validatable. Solves point 2 of #1776

This dramatically speeds up the comparison by reducing it to (effectively) order N from order ~N**2

In the case of the real user suite it gets the calculation down from 2.5+ hours (estimate based on the chunk I ran before giving up and assuming it can be read into memory) to 1 second.

@matthewrmshin - please review1
@hjoliver - please review2

[matthewrmshin]

Looks OK. Test OK.

[hjoliver]

Nice. For a simple suite with FAM1:succeed-any => FAM2:succeed-all and 100-member families, on my desktop box, I get a speed up from 90s to 15s, which isn't as dramatic as for your user suite, but still v good.

Review 2 - good, tests as working.