The notmuch worker followed suit in handling the dirInfo submission manually.
That removes the last user so we might as well remove the functionality.

There was an unused error value as well as unnecessary usage of the sort
interface. There should now be less copying so a bit better performance
in some cases.

This ensures that the directory info is up to date on events in the
maildir worker. This also sets up the initial dirinfo for other
directories and updates them when using built-in commands.

FS events are still only watched for the selected directory. This should
be changed in a future patch to watch other directories too in order to
cover UI updates for folders when an event occurs in a non-selected
folder.

When we fail to read a single message we don't need to fail the whole
search, just log the error and we can still get results back.

Apparently sending an event for every incoming messageInfo slows down
the application significantly.

Therefore this slows down the emmision rate, on the cost of being out of date
in some cases.