repair: Make repair_writer a shared pointer
repair: Make repair_writer a shared pointer

The future of the fiber that writes data into sstables inside
the repair_writer is stored in _writer_done like below:

class repair_writer {
   _writer_done[node_idx] =
      mutation_writer::distribute_reader_and_consume_on_shards().then([this] {
         ...
      }).handle_exception([this] {
         ...
      });
}

The fiber access repair_writer object in the error handling path. We
wait for the _writer_done to finish before we destroy repair_meta
object which contains the repair_writer object to avoid the fiber
accessing already freed repair_writer object.

To be safer, we can make repair_writer a shared pointer and take a
reference in the distribute_reader_and_consume_on_shards code path.

Fixes #7406

Closes #7430

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