Make type modelling Riak bucket stricter, and fix some Dialyzer warnings
Make type modelling Riak bucket stricter, and fix some Dialyzer warnings

Bucket field of state is assigned only in init that uses tuple.

Addressed Dialyzer warnings:
```
sumo_store_riak.erl:358: Invalid type specification for function sumo_store_riak:fetch_map/4. The success typing is (pid(),{binary(),binary()},binary(),[atom() | tuple()]) -> {'error',_} | {'ok',_}
sumo_store_riak.erl:381: Invalid type specification for function sumo_store_riak:update_map/5. The success typing is (pid(),{binary(),binary()},binary(),{'map',[{{_,_},_}],[{{_,_},_}],[{binary(),'counter' | 'flag' | 'map' | 'register' | 'set'}],'undefined' | binary()},[atom() | tuple()]) -> 'ok' | {'error',_} | {'ok',_} | {'ok',binary(),_}
```

inaka/sumo_db_riak