Refactor backup code

[Michael Hanselmann]

brackup builds its list of files before starting to backup them. With a large
number of files this will need a lot of memory. The code is all in a big, large
functions with a lot of closures. This patch series factors out some code of
the logic into classes and functions. My idea was to add another class which
would gather files while backuping them once this preparation work was done.