Add ApplicationProtocolConfig.DISABLED

Motivation:

When ALPN/NPN is disabled, a user has to instantiate a new
ApplicationProtocolConfig with meaningless parameters.

Modifications:

- Add ApplicationProtocolConfig.DISABLED, the singleton instance
- Reject the constructor calls with Protocol.NONE, which doesn't make
  much sense because a user should use DISABLED instead.

Result:

More user-friendly API when ALPN/NPN is not needed by a user.