Add backend_status command to riak-admin.
Add backend_status command to riak-admin.

Fixes: az722

Add a backend_status command to riak-admin that fetches the status of
the backend for each available vnode. The vnode index, the backend,
and the data returned from calling the backend module's status
function are displayed.

elkingtonmcb/riak,amyvmiwei/riak,authbox-lib/riak,basho/riak,yangchengjian/riak,biddyweb/riak,laurenrother/riak,xuejianhai/riak,GabrielNicolasAvellaneda/riak,DavidAlphaFox/riak,planobe/riak,planobe/riak,glickbot/riak