streaming: opt-in to compacting the stream

Use locally generated compaction time on each node. This could lead to
different nodes making different decisions on what is expired or not.
But this is already the case for streaming, as what exactly is expired
depends on when compaction last run.

streaming/stream_transfer_task.cc

@@ -70,7 +70,7 @@ struct send_info {
         , cf(tbl_)
         , ranges(std::move(ranges_))
         , prs(dht::to_partition_ranges(ranges))
-        , reader(cf.make_streaming_reader(cf.schema(), std::move(permit_), prs, {}))
+        , reader(cf.make_streaming_reader(cf.schema(), std::move(permit_), prs, gc_clock::now()))
         , update(std::move(update_fn))
     {
     }