v0.14.0: Guaranteed block time, integration fixes, many preimages, timer optimizations

Major improvements since v0.13.0:

• Agora will now produce blocks at the right interval, even if empty;
• The agora_preimages_gauge stats entry has been renamed to agora_preimages_counter;
• Fixed the VoterCard signature scheme to only hash once, not twice;
• Fixed QR admin methods to not escape the returned string;
• Nodes will now generate a large (5M) of pre-images on startup,
  allowing them to stay validators for longer timer before switching UTXOs;
• Various timers used for recurring tasks have been optimized to only fire when necessary,
  instead of a fixed regular interval;
• Agora will now longer automatically sign blocks when catching up, to avoid potential double signatgure issues;