Request frequency specifications not enforced

According to the [documentation](https://developers.google.com/safe-browsing/v4/request-frequency):

> Both the fullHashes.find response and threatListUpdates.fetch response have a minimumWaitDuration field that clients must obey.

And

> Automatic back-off applies to both the fullHashes.find response and threatListUpdates.fetch response.

This issue was first raised in [#33](https://github.com/google/safebrowsing/issues/33), and marked as resolved in [#49](https://github.com/google/safebrowsing/pull/49), however this only addresses the database update, and not `fullHashes.find`.

On a different note, enforcing `minimumWaitDuration` seems reasonable for both methods, but would back-off mode make sense for `fullHashes.find`? eg. do we really want to prevent any further hash confirmation for at least 15 minutes for a single 4xx or 5xx response?