Eclipse Californium (Cf) CoAP Framework 2.6.0

Currently the final planed 2.x release with minor extensions and bugfixes.

Scandium:

• bugfix wrong certificate verify for empty client certificate.
• use client-hello's version in hello-verify-request instead of 1.0 (differs from RFC6347 recommendation)

Californium:

• customize serializer

Examples:

• Extend benchmark to support NON requests