pipeline: Batch items queued/downloaded updates.

Previously, when wpull processed a page containing many URLs, we'd fire
off many HINCRBYs.  Furthermore, redis-py operations block until the
Redis command completes (not redis-py's fault -- this is part of the
Redis protocol), so we'd also block wpull processing for a good long
while (single-thread, Python global interpreter lock, etc).

This commit only updates counts once per handled response, which yields
fewer network transmissions and fewer blocking operations.