Fix issue with batchHandler not being called without also using handler

[danielbcorreia]

Motivation:

The Kafka Client API provides two ways of processing records, using the handler and batchHandler methods. It turns out that batchHandler cannot be used independently in the current implementation, since a call to handler (even if the function itself does nothing) is required in order for the batchHandler to be called.

The same issue in #170.

This fix changes the following:

• null check on schedule now takes into account both handler types
• to preserve the same guarantees provided for handler, the batchHandler reference is now captured in schedule and passed down to the run method.
• the test for batchHandler no longer calls handler (verified that this makes the test time out without the actual fix)

The name multiHandler on the schedule method is not great but I didn't want to shadow the field name, I'm open to suggestions.

[danielbcorreia]

Closing this and opening a new PR with the correct branch