Add #riak_core_fold_req_v2 record, use new options for AAE tree building

NOTE: This PR is incomplete.  I've created it for ease of discussion this week in Cambridge.

Add `aae_reconstruction` annotation to the fold request sent by
riak_kv_index_hashtree:fold_keys().

Also add not-forwardable annotation to the AAE hash tree fold,
to address GitHub issue #621.

If riak_kv_vnode:handle_command() gets a #riak_core_fold_req_v1 command,
that command is converted to a v2 command before continuing.  The
?FOLD_REQ.opts proplist is appended to the end of the `Opts` proplist
that is passed to the KV storage manager's Mod:fold_objects/4 function.