fix race condition on node-startup with proxy shutdown check

module not yet registered with riak core but proxy_sup can have childspecs
for app's vnode supervisors before this happens
remove unneeded change to riak_core_ring:all_next_owners

its implementation was a bit misleading with the changes and was only
needed to support one part of the codebase (starting extra vnode proxies
when ring expands)
add eqc test for riak_core_ring:future_index