Add riak_kv_ensemble_backend:handle_down/4

Have the backend monitor the vnode and vnode proxy. The peer will
receive 'DOWN' messages and call riak_kv_ensemble_backend:handle_down/4
as a result.

Part of a fix for basho/riak_kv#985
Fix MOD_MAP when datatype update request includes return_body=true
Merge pull request #987 from basho/bugfix/sdc/dtupdate-mod-map

Fix MOD_MAP when datatype update request includes return_body=true

Reviewed-by: russelldb
Fix AAE on/off detection

riak_kv/anti_entropy takes a value of the form {on|off, Opts}, not on
off.
Correct discrepancy between do_get_term and case clause in do_delete

This was causing node shutdown when running the Java client
integration tests. On a small scale, this would be fine (one vnode
crashing would start back up), but in massive-delete scenarios entire
clusters could go down. We should heed the TODO message in do_get_term
and handle this holistically, but for now this should prevent the
vnode from crashing.
Merge pull request #993 from basho/bugfix/fix-ensembles-aae-detection

Fix AAE on/off detection

Reviewed-by: coderoshi
Merge pull request #992 from basho/bugfix/sdc/notfound-badspec

Correct discrepancy between do_get_term and case clause in do_delete

Reviewed-by: reiddraper
Merge pull request #990 from basho/bugfix/monitor-vnode-proxy

Add riak_kv_ensemble_backend:handle_down/4

Reviewed-by: lordnull
SC ensemble peers should wait for riak_kv service