detect and handle changed responses in poll. for #9, #321

not quite done because this stops using the cached seen responses. i *think* i can get that back, but i'll have to store the full json of seen responses. :/
poll: cache recently seen full response objects in memcache

fixes #9 and fixes #321, hopefully. finishes what i started in 2fe957f86025a4ec947decd6f233cc256d6688f6.