CAF::FileWriter should use external diff command for large files

[gombasg]

We've just run into an issue with the AII dhcp plugin, where Text::Diff, invoked by CAF::FileWriter, took almost 3 hours to compute just a few lines worth of diff. Since Text::Diff is declared to be slow on large files, it would be better to call an external diff command if the inputs are large (e.g., more than 1000 lines).

Currently, the diff is computed even if it is then not going to be logged/displayed, which is inefficient. Fixing that would also be useful.