Support long polling, save a lot of cost on AWS

thumbd server was not using Long Polling due to which it was
requesting SQS every 76ms (on my server). This means that each
day it was making over a million requests. Which is a lot of
cost. About $0.5 a day for each instance of thumbd.
Now you can specify --wait_time=N where N is seconds from 1 to 20
and it'll long poll thus saving the number of requests by a great factor.