forwardable reader: implement fast_forward_to(position_in_partition)
forwardable reader: implement fast_forward_to(position_in_partition)

Instead of throwing std::bad_function_call. Needed by the foreign_reader
unit test. Not sure how other tests didn't hit this before as the test
is using `run_mutation_source_tests()`.

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