Bugfix/eqc process leak

Fix process leak found in #761 

- Stop riak_cs_get_fsm and riak_cs_dummy_reader processes after single command sequence
- Refactoring

Increasing beam.smp's memory size can be seen by, for instance:

```
$ ./rebar eunit skip_deps=true compile_only=true && \
    ERL_LIBS=$ERL_LIBS:deps erl -pa .eunit
> spawn(fun() -> riak_cs_get_fsm_eqc:test(1000) end).
```