Don't fail if no dry-run setting has been specified

Somehow we failed if no dry-run setting had been specified in the config
file. This got caught thanks to extending the test suite with a stock
configuration.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Fixed the deadlock bug when sync'ing a IMAP folder in singlethread mode
Added support for email date synchronization with the file timestamp, through the repository "syncdate" option.

When using the (new) "syncdate" option in a Maildir repository, the local file will have the mtime/atime matching the messages' date.
This is useful if you are doing some processing/finding on your Maildir (for example, finding messages older than 3 months), without parsing each file/message content.
Without the option, the local file will have their timestamp set to the time offlineimap ran.

You likely want this option when you bootstrap your initial synchronization, or if you run offlineimap very sparsely.