Adds a SortedArray to replace the cranky deferred cache.
Add a test that demonstrates the broken cache.
Rip out the cache and add the SortedArray.
Actually use SortedArray, and ensure elements with the same run_at time aren't deleted prematurely.
Use the mysql adapter by default.
Merge branch 'fix_deferred_tasks'

Conflicts:
	lib/tom_queue.rb
	lib/tom_queue/deferred_work_set.rb
Adds a quick benchmarker tool I used to test deferred_work_set optimisations