Fix data race in the topology relation cache

[LouisParkin]

SubmitRelation parks relations whose endpoints are not yet known in possibleRelations. The collectors goroutine and the correlator goroutines append to that slice concurrently, and the WaitGroup around the append did not exclude them, so relations could be lost from a topology snapshot. The cache now uses a mutex, and CorrelateRelations reads a snapshot of the slice under it.

The new test submits relations from 8 goroutines. Before the fix it reports data races under -race and loses relations; after the fix, the package tests pass under -race.

Fixes #536

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