mutation_partition: introduce mutation_querier
mutation_partition: introduce mutation_querier

mutation_querier is a streamed_mutation consumer that adds the mutation
content to query::result.

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>

scylladb/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,kjniemi/scylla,kjniemi/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,avikivity/scylla