Mail Finger Print defaults to "delete mail" even if there are no fingers prints.

JIRA 3.13 added new "finger print" handling to the mail service code.

The idea was that if JIRA has had its "fingers on an email before" then we dont want to continue to handle the message.

HOWEVER in doing this we have also change the default behaviour of the "deleteEmail" flag. It nows will delete an email by default unless told other wise

This might not be a wise choice as proven by JRA-15550

Also its possible not to have any finger prints on an email and still the default "delete" behaviour

We need to review again the intention of the finger print code and possible change the default "delete" back to false unless we are really sure.

This issue is a placeholder issue to perform that work and ensure email handling is still rock solid