This commit lets etcdserver check permission during its log applying
phase. With this change, permission checking of operations is
supported.
Currently, put and range are supported. In addition, multi key
permission check of range isn't supported yet.
Current etcdserver tries to return result.resp even if result.err is
not nil. A situation of result.resp == nil and result.err != nil can
happen and it results an error like below:
18:49:57 etcd1 | interface conversion: proto.Message is nil, not *etcdserverpb.PutResponse
This commit lets the functions return result.err if it is not nil.
This commit implements Authenticate() API of the auth package. It does
authentication based on its authUsers bucket and generate a token for
succeeding RPCs.
