add riak_ensemble_peer:tree_info/1 for debugging
Enhance replace_members_test

The prior version of this test replaced the root ensemble members:
root/2/3 -> 4/5/6.

This version adds another replacement, from 4/5/6 back to root/2/3.
The complete transition in this test is thus:
root/2/3 -> 4/5/6 -> root/2/3.