Implement key count estimation via AAE trees

This pull-request updates `riak_kv_index_hashtree` to provide an API around the new AAE-based key count estimation feature added to the `hashtree` module in `riak_core`.

This provides a mechanism to estimate the number of keys in a given K/V vnode based on its AAE trees.

This estimate is used to enable an improvement in the operation of AAE-based fullsync replication in Riak Enterprise.

Sibling pull-requests: basho/riak_core#633, basho/riak_repl#623