new: Synchronize maildir flags eagerly.

Because flag synchronization is stateless, it can be performed at any
time as long as it's guaranteed to be performed after any change to a
message's filename list.  Take advantage of this to synchronize tags
immediately after a filename is added or removed.

This does not yet make adding or removing a message atomic, but it is
a big step toward atomicity because it reduces the window where the
database tags are inconsistent from nearly the entire notmuch-new to
just around when the message is added or removed.