better solution for skipping notifications

The previous solution of cloning was a bad one. Apparently cloning
wastes CPU like crazy. For big objects it takes seconds!
The new solution keeps track of files hashes and skips notifications if
file content did not change.
There are also no need for notifications from the publishing path. Those
are called only upon file load.
The tests are also had a flaw. Rebus instances should be different
(disable signleton mode) to check how changes are go from one rebus
instance to another.