Fix performance regression - network calls too early

@sdboyer

From @sdboyer on April 4, 2017 1:21

Fix the performance regression arising from how early we go out over the network to verify a source's upstream existence: sdboyer/gps#196 (comment)

This may be a nontrivial fix, as it would create a situation where it's possible for a source that initially looks good to become bad later.

Copied from original issue: sdboyer/gps#207

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