Use apdb_marker to test for association completion.

This should be faster (and is definitely simpler) than loading
VisitInfo and querying the database directly.

python/lsst/ap/pipe/ap_pipe.py

@@ -209,8 +209,7 @@ def runDataRef(self, rawRef, templateIds=None, reuse=None):
                     "matters, please clear the association database and run "
                     "ap_pipe.py with --reuse-output-from=differencer to redo all "
                     "association results consistently.")
-            if "associator" in reuse and \
-                    self.apdb.isVisitProcessed(calexpRef.get("calexp_visitInfo")):
+            if "associator" in reuse and calexpRef.datasetExists("apdb_marker", write=True):
                 message = "Association has already been run for {0}, skipping...".format(calexpRef.dataId)
                 self.log.info(message)
                 associationResults = None