Performance optimization for object listing requests using a prefix

* Use prefix in start key calculation when appropriate to avoid
 iterating over unnecessary keys that lexicographically sort less
 than the prefix. The make_start_key_from_marker function in
 riak_cs_list_objects_fsm_v2 is renamed to
 make_start_key_from_marker_and_prefix.
* Terminate processing early when a prefix is specified and a key is
 reached that lexicographically sorts greater than the prefix, but
 does not begin with the prefix. The results come back in sorted key
 order so it can be assumed that once any keys that begin with the
 prefix are processed it is appropriate to cease processing without
 requesting any more data. This is accomplished by the addition of
 the reached_end-of_keyspace/4 function to the
 riak_cs_list_objects_fsm_v2 module.
* Silence extraneous console logging in riak_cs_list_objects_fsm_v2_eqc test