Add a ContactHarvester arg to ImapDB.Folder.create_or_merge_email and
call it on the resulting email. Add and update a new harvester
property to MinimalFolder and pass that in as needed.
Replaces harvesting code in ImapDB code so as to be stand-alone and
reusable. Implement harvesting policy to only harvest from a set of
whitelisted special folder types (so junk and trash is not harvested)
and only harvest valid, non-spoofed, addresses.
