Add support for get_watermark_offsets

[mattusifer]

This PR adds a new consumer function - get_watermark_offsets - which leverages the get_watermark_offsets librdkafka binding to return the last known watermark values for a topic/partition. This is essentially a much more efficient version of query_watermark_offsets, which requires a network call to the broker, with some caveats (described in the docs for the get_watermark_offsets method).