Use dict to speed up record search

This dict is created earlier, presumably to speed up this exact lookup, but it was using the list form instead, requiring a linear scan for each item.

octodns/provider/cloudflare.py

@@ -585,7 +585,7 @@ class CloudflareProvider(BaseProvider):
         changed_records = {c.record for c in changes}
 
         for desired_record in desired.records:
-            if desired_record not in existing.records:  # Will be created
+            if desired_record not in existing_records:  # Will be created
                 continue
             elif desired_record in changed_records:  # Already being updated
                 continue