fetchTopicOffsets should return low watermarks

[sklose]

enhances the admin client's fetchTopicOffsets function to return the low + high watermarks for the topic. this is related to #316

I kept the existing offset field so it's backwards compatible and added high + low (offset and high are equivalent)

[Nevon]

Took me a minute to understand that this was mutating offsets. How about moving this into the map above instead? Something like:

const { partitions: lowPartitions } = low.pop()
const offsets = partitions.map(({ partition, offset }) => ({
  partition,
  offset,
  high: offset,
  low: lowPartitions.find(({ partition: lowPartition }) => lowPartition === partition).offset
}))

This is assuming that all partitions present in high would also be present in low, but I see no reason they wouldn't be.

[sklose]

good point, fixed

[Nevon]

Great, thanks!