backend: remove all files early

We encountered a problem with late files removal. Those removals were clashing with postinst scripts. Postinst script expect to be run on fully updated system and having leftover files from older version was not expected. This had effect like removal of files that were generated by postinst or postrm scripts or uninstalled plugins being loaded to applications on service restart by postinst.

This just simply reuses existing facility to remove files early during files merge. Instead of using it to remove files that collide with directories we use it to remove all abundant files. It is not an ideal solution but we are unable to drop previous approach because of backward compatibility with previous journals.

Fixes #260 (closed)