Current etcd code uses the string literals ("token", "authorization")
as field names of grpc and swappger metadata for passing token. It is
difficult to maintain so this commit introduces new constants for the
purpose.
Like the previous commit 10f783efdd12, this commit lets grpcproxy
forward an auth token supplied by its client in an explicit
manner. snapshot is a stream RPC so this process is required like
watch.
This commit lets grpcproxy handle authed watch. The main changes are:
1. forwrading a token of a new broadcast client
2. checking permission of a new client that participates to an
existing broadcast
