Make eternal_watch use modifiedIndex, not etcd_index
Make eternal_watch use modifiedIndex, not etcd_index

Contrary to what advised in the etcd documentation as of now, the
correct way to handle a request for eternal watch is to increment on the
modifiedIndex and not on X-Etcd-Index.

jplana/python-etcd,aziontech/python-etcd,jlamillan/python-etcd,projectcalico/python-etcd,j-mcnally/python-etcd,kireal/python-etcd,sentinelleader/python-etcd,jlamillan/python-etcd,mbrukman/python-etcd,vodik/python-etcd,thepwagner/python-etcd,vodik/python-etcd,ocadotechnology/python-etcd,thepwagner/python-etcd,mbrukman/python-etcd,dmonroy/python-etcd,aziontech/python-etcd,dmonroy/python-etcd,projectcalico/python-etcd,ocadotechnology/python-etcd,jplana/python-etcd,j-mcnally/python-etcd,kireal/python-etcd,sentinelleader/python-etcd