Ensure that everything gets backed up at least once

We now schedule a backup pass for any new application we see (at boot or
package-install time) for which we have never performed a backup.  The
bookkeeping is a log file with the names of all the packages we've successfully
completed a backup pass on, maintained with synchronous journal-type behavior.

Also, make a bunch of private fields package-scoped to permit cheaper access
from the various worker / binder threads that run under the Backup Manager's
aegis.