feeder: empty the API cache before feeding each form

The cache grows too much without it.

wcs_olap/feeder.py

@@ -395,6 +395,7 @@ class WcsOlapFeeder(object):
                 self.do_schema()
                 self.do_base_table()
             for formdef in self.formdefs:
+                self.api.cache = {}
                 try:
                     formdef_feeder = WcsFormdefFeeder(self, formdef, do_feed=self.do_feed)
                     formdef_feeder.feed()