raft: msg.Denied -> msg.Reject

Change the field name because it has msgDenied already.
Merge pull request #1224 from unihorn/149

raft: msg.Denied -> msg.Reject
*: expose etcd-index in watch requests

This adds a StartIndex field to the Watcher interface, which represents
the Etcd-Index at which the Watcher is created.

Also refactors the HTTP tests to use a table for most handleWatch tests