Add helper function to group queries into batches by host / shard

[mmatczuk]

This is in the context of scylladb/gocql#90.
It would be nice to add a dbutil that would partition batches to hosts and shards.

[mmatczuk]

There exists prior work that is over complex and over specialized in my view #167.

One can see there there is a need for grouping queries to batches.
It should be flexible to allow extending it to support other use cases.