dialyzer fixes for cluster metadata

addresses issue #392

* fix some typespecs in dvvset, riak_core_metadata and metadata manager
* change some opaque types to regular ones
* don't reuse `remote_iterator` as message to manager process since its
a record too
* fix typo in exchange_fsm that didn't harm anything

This PR does not fix the issues found by dialyzer in
riak_core_metadata_exchange_fsm (see [here](https://gist.github.com/jrwest/760d9934c5d64d30252e)). Dialyzer isn't able to figure
out we are doing the "right thing" with the iterators here.