Update K/V hashtrees to support key estimation

This commit 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.

Authored-by: Mikael Lixenstrand <mikael.lixenstrand@erlang-solutions.com>
Rebased-by:  Joseph Blomstedt <joe@basho.com>
Make AAE key estimation API handle illegal IndexN

This commit changes riak_kv_index_hashtree to return not_responsible
when provided an illegal IndexN in estimate_keys/2. Prior to this
commit, an invalid IndexN would cause the hashtree process to crash.

Authored-by: Kresten Krab Thorup <krab@trifork.com>
Rebased-by:  Joseph Blomstedt <joe@basho.com>
Merge pull request #1030 from basho/feature/aae-estimate-keys

Implement key count estimation via AAE trees

Reviewed-by: engelsanchez