The RPC call returns only the updated Challenge object instead of the
whole authz, because the VA shouldn't be allowed to update any of the
other fields of the Authorization object.
This allows us to call the Google Safe Browsing calls through the VA.
If the RA config's boolean UseIsSafeDomain is true, the RA will make the RPC
call to the VA during its NewAuthorization.
If the VA config's GoogleSafeBrowsingConfig struct is not nil, the VA
will check the Google Safe Browsing API in
VA.IsSafeDomain. If the GoogleSafeBrowsingConfig struct is nil, it will
always return true.
In order to actually make requests, the VA's GoogleSafeBrowsingConfig
will need to have a directory on disk it can store the local GSB hashes
it will check first and a working Google API key for the GSB API.
Fixes#1058
Kane York
Roland Bracewell Shoemaker
Jacob Hoffman-Andrews
Jeff Hodges