Optimize riak_kv_entropy_info:exchanges/2

This commit changes `riak_kv_entropy_info:exchanges/2` from being an `O(num_primary_vnodes)` operation to an `O(1)` operation. Since `exchanges/2 is` used heavily for `riak_ensemble` based syncing, this change significantly improves `riak_ensemble` usage for large rings.

Sibling pull-request: basho/riak_ensemble#21
Together with above sibling pull-request, resolves issue basho/riak_ensemble#15