Make it possible to disable AAE tree expiration
Make it possible to disable AAE tree expiration

AAE tree expiration is now disabled if riak_kv/anti_entropy_expire is
set to the atom 'never'.

This commit also adds riak_kv_index_hashtree:clear/1 that can be used
to manually clear a hashtree, including when automatic expiration has
been disabled.

andytill/riak_kv