streamed_mutation: Allow setting buffer capacity
streamed_mutation: Allow setting buffer capacity

Needed in tests to limit amount of prefetching done by readers, so
that it's easier to test interleaving of various events.

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