Add gen.moment, a yieldable object that resumes after one IOLoop iteration.

Add gen.moment, a yieldable object that resumes after one IOLoop iteration.

Use this between requests in HTTP1ServerConnection to keep one connection
from monopolizing the IOLoop when there are pipelined (or just fast) requests
available.  This was causing increased variance in benchmarks using "ab -k"
(it also increased throughput, for reasons that are not yet clear, so this
change reduces observed speed in these benchmarks).