Add Backlog method signature to Consumer interface

[JeanMertz]

This allows for processors to determine their mode of operation, based on the messages still waiting to be consumed.

For example, if a processor continuously consumes messages, and periodically produces messages based on the previously consumed messages, it might be desired to have the producer only trigger if the backlog of the consumer is small enough. This allows for dynamically catching up with a larger backlog during peak-hours.

The method signature is straightforward:

Backlog() (int, error)

Stream client implementations that have no concept of message persistence or consumer identity, always return 0. The error will be non-nil if any error occurred while fetching the required data.