Significant improvements to the proximity map.

 * Removed the workaround that involved truncating the views cache.
 * Fix [#321291]
 * Behavior change: Exposed proximity map filters now have correct field names. This allows optional exposed proximity maps to work. (Otherwise, views would not be able to see that the filter was actually filled in.)
 * Work around a Views 1 bug that failed to persist filter settings properly when editing a view.