Buffer usage

Minor refactoring @xla  @juliusv 
While I think this isn't explicit in our Go style guide, it's quite common in Go to give short names to variables that are repeated very often and whose meaning is clear from immediate context. In this case, something like `bm` instead of `backupManager` will IMO actually read better below, since it will immediately jump at your eye as the instance variable, and you still will have no problem finding its meaning.

This is also what you'll find when looking at any method in the standard library.