Process to remove unused handlers over time

This bug was originally reported by Chris Lamb (lamby@debian.org) in Debian bug #833603:

As we fix various issues in packages and toolchains, the number of
handlers in strip-nondetermism should eventually (!) drop to zero. 

I think we can speed this up by getting some visibility on which
handlers are actually being used and pro-actively removing them
when they drop below a certain level.

One way we could do this would be to log that we applied $handler
on $filename then that could be parsed by external tools.

If it then appears more sensible to fix each package individually
(ie. due to a low number of packages requiring it) or to attack the
root cause of the problem (eg. some kind of toolchain fix), we can
eventually remove the handler.