multishard_mutation_query: add support for keeping stats for discarded partitions
multishard_mutation_query: add support for keeping stats for discarded partitions

In the next patches we will add code that will have to discard some of
the dismantled partitions/fragments/bytes. Prepare the
`dismantle_buffer_stats` struct for being able to track the discarded
partitions/fragments/bytes in addition to those that were successfully
dismantled.

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla