mutation_fragment: allow creating clustering row in place
mutation_fragment: allow creating clustering row in place

Moving clustering_row is expensive due to amount of data stored
internally. Adding a mutation_fragment constructor that builds a
clustering_row in-place saves some of that moving.

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