also remove the continuation support from the queue, otherwise
we may end up sequential scanning the entire table to get
a few rows at the end.
then, in the core, instead of looping both to get a big enough
batch inside of the queue, as well as outside of it to ensure
we consume the whole queue, just get a single batch at a time.
also, make the queue size configurable because we'll need to
do some tuning in production.
Change-Id: If1a997c6012898056ace89366a847c4cb141a025
44433f38be