Add configurable delay for Google geocoding to avoid 620 errors

The function google_geocode_location() in geocoding/google.inc currently returns NULL if Google's geocoding server returns any type of error.

Error code 620 usually indicates that the user "has submitted too many [geocoding] requests in too short a period of time" (http://code.google.com/apis/maps/documentation/reference.html#GGeoStatus...). In my experience, the Google servers issue this code if they receive more than a few (1 to 5?) requests per second from a given site. It's easy for a popular site to exceed Google's velocity threshold, and even easier in the case of bulk import operations that invoke node_save(). Retrying the operation after a one second wait will usually succeed.

So this is a fairly common "soft" error that's easily rectified by retrying the operation. Unfortunately the functions that invoke google_geocode_location() (notably including node_save()) treat this as an unrecoverable condition and lat/long values of 0.0000 are stored in the Location node. This obviously causes problems for users and developers for which there's no recourse at this time.

I suggest adding a configuration parameter to the Location module's Geocoding options (admin/settings/location/geocoding), specifying the number of times to retry a 620 error, with an implied one-second sleep between tries. The default value for this retry parameter should be zero so as not to alter the behavior of existing installations. An upper limit of, say, 30 is probably also in order.

Note: Other Drupalers have alluded to this problem but not written it up explicitly. See: http://drupal.org/node/301275#comment-2153602 and http://drupal.org/node/317159#comment-2145356.