An EntityProcessor instance which can index emails along with their
attachments from POP3 or IMAP sources. Refer to http://wiki.apache.org/solr/DataImportHandler for more details. This
API is experimental and subject to change
This method is called when it starts processing an entity. When it comes
back to the entity it is called again. So it can reset anything at that point.
For a rootmost entity this is called only once for an ingestion. For sub-entities , this
is called multiple once for each row from its parent entity