Move args from open_async to ctor and use these to determine and store
DB file and attachment paths as properties. This allows constructing
the DB instance up front and markig it as non-nullable, simplifies
calling patterns and hence allows removing one use of the deprecated
Engine singleton, and by moving in the local data deletion
implementation from ImapEngine.GenericAccount, also allows removing a
static helper method.
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.
Updating the unread count after opening a folder and finding email that
has an unexpected unread status messes up the count obtained from the
server, which has already taken these messages into account.
Here, both the main normalisation process and the email flag updater are
prevented from adjusting the unread count for a folder when they
encounter email that are new and unread, or have an unread status
different from what was last seen by the engine.
See #213
