managed_bytes: Declare copy constructor as allocation point
managed_bytes: Declare copy constructor as allocation point

Because of the small size optimization, not all copies will call the
allocator, so allocation failure injection may miss this site if the
value is not large enough. Make the testing more effective by marking
this place explicitly as an allocation point.

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