Reactivate RHD

[gnunicorn]

Since 1.0 we are shipping with a pluggable consensus mechanism. In order to make this possible, we had to move out the previously installed rhododendron-PBFT, which has not been reactivated since. However, in order to proof that our consensus truly is pluggable we want to provide more than one algorithm to choose from. Let's reactivate the substrate-consensus-rhd crate and make rhd a possible choice for node. In order for that to be possible, one must:

• take a close look at the substrate-consensus-rhd crate and remove old, unnecessary cruft, adapt to latest API changes and make it compile and the tests pass again
• adapt the rhd-bindings to use the new messaging layer and system (introduced in Consensus Engines Implementation: Aura #911)
• provide a rhd-Verifier to the ImportQueue-Mechanism
• change node (-template) to provide an example how to use rhd rather than Aura+Grandpa