Move the _notmuch_message_sync from private to public interfaces

The idea here is to allow internal users to see a non-synced message
object, (for example, while parsing a message file and incrementally
adding terms, etc.). We're willing to take the care to get the
improved performance.

But for the public interface, keeping everything synced will be much
less confusing, (reference lots of sup bugs that happen due to
message state being altered by the user but not synced to the database).