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.
Merge pull request #943 from basho/bugfix/optimize-many-ensembles

Optimize riak_kv_entropy_info:exchanges/2

Reviewed-by: andrewjstone