Suppress "Source contains property not mapped in the target" when appropriate

[andrus]

A message "Source contains property not mapped in the target" was intended to warn when the extractors have unexpected attributes, but more often than not these attributes are added intentionally and are processed by stage listeners. Would be nice to suppress warnings in the later case . Perhaps a special tag on <attribute> in extractor indicating that this attribute is needed, even though it is not associated with the model?

c.n.l.m.r.t.c.CreateOrUpdateMerger: Source contains property not mapped in the target: db:is_online. Skipping...