Add in option to disable max requests killing if the user chooses

Added in an if call to lib/unicorn/worker_killer.rb to check if the `use Unicorn::WorkerKiller::MaxRequests, 0, 0` is set. If so it bypasses the MaxRequest check.

We found we needed this on a larger site (~40k concurrent sessions) as requests quickly built up and it was really the memory we were interested in anyway. Limited the number of unicorn deaths without removing the overall usefulness of the gem.