transactions & fix-p2p & feat-pp-hash & ci

• peers are removed due to timeout
  • -> combat by using ping and keep_alive
• cli qol features
  • show time
  • fix block id
  • add hostname to signature
  • show only last 10 entries with ls
• add identify, ping & rendezvous protocol
• store and load blockchain to/from json file
  • autosave chain
• pp-hash @oliver.hanikel
  • automatic mining difficulty
• add transactions
  • UTXO
  • signing
  • base64 representation of data
• ci with caching