Change modification-tracking mechanism to use counter instead of

Change modification-tracking mechanism to use counter instead of
boolean flag.  This is necessary if more than one process is
simultaneously tracking modifications.

Sketch new synchronization interface.  New interface consists of three
procedures: FOLDER-SYNC-STATUS determines the synchronization
relationship of the folder cache with the persistent folder;
SAVE-FOLDER saves any cached changes to the persistent folder;
DISCARD-FOLDER-CACHE discards all cached information.