Previously, synchronizing folders caused a jump in CPU, so work was
done to minimize activity.  Optimizations to folder normalization
has reduced the CPU load, so it's okay now to allow flag watching
while the background folder is open.

This is not a substitute for an architected solution: periodic
flag synchronization with folders, not merely when changes in contents
(additions/removals) are detected, but this helps some, especially
with high-traffic folders.