Reduced memory usage when there are a large number of operations in the queue

Added autorelease pools to some areas where Instruments measured high
memory usage (when many operations were in the queue).

Another area was the initialization of AFURLConnectionOperation,
specifically the creation of a to-memory output stream. This has been
changed so that the output stream is now created lazily.
Coincidentally, this also guards against the programmer error of
setting the output stream to nil.