Harden ByzCoin

[ineiti]

The following issues should be implemented to harden byzcoin:

• Limit access to the node dedis/cothority#2204 / Limit allowed connections dedis/onet#620 - Amplification attack: limit access to node-node communication
• More validity checks for ClientTransactions dedis/cothority#2205 - block flooding: validity checks for ClientTransactions
• Limit service endpoints dedis/cothority#2207 - flooding and CPU exhaustion: limit service endpoints
• Simplify BEVM contracts dedis/cothority#2206 - cleanup: simplify BEVM contracts
• Remove calypso protocol #12 - lesser attack surface: remove calypso protocol
• Disable blscosi protocol #13 - lesser attack surface: disable blscosi protocol