mutation_reader: combined_reader: attach stream_id to mutation_fragments
mutation_reader: combined_reader: attach stream_id to mutation_fragments

The fragment producer component of the combined reader returns a batch
of fragments on each call to operator()(). These fragments are merged
into a single one by the fragment merger. This patch adds a stream id to
each fragment in the batch which identifies the stream (reader) it
originates from. This will be used in the next patches to associate
range-tombstone-changes originating from the same stream with each other.

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