Fixed problems associated with the `GeometryField` SRID cache: it is now keyed...

Fixed problems associated with the `GeometryField` SRID cache: it is now keyed by the connection alias rather than the spatial backend name (e.g., `spatial_ref_sys` tables may have different entries, even if backend is the same), explicitly pass in connection alias to `SpatialRefSys` query to ensure correct entry is retrieved, and removed unused code from `GeometryField.get_distance`.  Thanks, Josh Livni, for the bug report.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12258 bcc190cf-cafb-0310-a4f2-bffc1f526a37