AllUrlsValid validator should use grequests to parallelise checks

[jace]

Rather than checking one URL at a time, as at present, which blocks on I/O, the AllUrlsValid validator should use grequests to parallelise the requests and check one as it returns.

The validator also checks the same URL twice if it appears twice, depending on the cache to prevent a second lookup. Since the cache isn't guaranteed when checking requests in parallel (and as an externally hosted cache is also a network call away), AllUrlsValid should also de-dupe URLs before doing the checks.