Do not display autovacuum workers in the sessions graphs

I think it makes sense to keep them only on the vacuum graphs (database
        objects view)
Without this, we get very long "transactions" and xids displayed in the
main graph, and it's confusing

powa/sql/views_graph.py

@@ -546,6 +546,7 @@ def BASE_QUERY_PGSA_SAMPLE(per_db=False):
         AND pgsac.srvid = %(server)s
       ) AS pgsa_history
       {extra}
+      AND backend_type <> 'autovacuum worker'
     ) AS pgsa
     WHERE number %% ( int8larger((total)/(%(samples)s+1),1) ) = 0
 """.format(extra=extra)