Being in the batchconsumer package means it will work for anything
using KCL, but lambdas that subscribe to these log streams do not use
batchconsumer at all; instead they invoke the splitter package
directly. As such, if we want this functionality to be available to
lambda log consumers, it can't be in batchconsumer.
There are no functionality changes here, just moving code from an
unexported method in one place to an exported function in another
place. The tests also get moved along with it.
