Minor changes to riak_kv_index_hashtree based on code review

Remove now-dead handle_cast clauses for insert, insert_object, and
delete in riak_kv_index_hashtree that were moved to handle_call in
order to introduce backpressure.

Make explicit the requirement that the IndexNs parameter passed to
start/3 and start_link/3 must be a non-empty list. Various code in
riak_kv_index_hashtree is written assuming this condition.