Add option to store last time a file was used in a build

In tor-browser-build, we can clean old build files by running make clean, which will keep only the files used in the current build.

There are however some problems with that:

• running make clean is slow as it needs to compute all the file names used in a build, for all the branches/series we are using.
• when working on different bugs at the same time, running make clean will clean build files created by branches that have not been merged yet.

Instead, it would be nice to be able to clean files that have not been used in the last X days. To allow that, rbm should have an option to store somewhere a timestamp of when each input files have been used in a build for the last time.