riak_cs_gc_d used to have bunch of complex states not only name of
states exposed in gen_fsm, but also many elements in gc_d_state
record. This commit extracts operator-facing states such as idle,
paused, running from riak_cs_gc_d. riak_cs_gc_d is not a long-running
worker under supervision tree any more, but riak_cs_gc_manager is.
The lifecycle of riak_cs_gc_d is now as long as one GC batch - if the
GC has finished then riak_cs_gc_d process terminates.

See comments in both riak_cs_gc_d.erl and riak_cs_gc_manager.erl for
detail state diagram and description.

For now, pause and remove can be replaced with stopping and setting
interval. This simplifies two FSMs drastically in clean way.

…mments