Merge branch 'master' into refactor/substrate-state-machine-generic

* master: (86 commits)
  Make contract a separate runtime module (#345)
  Version bump (#450)
  DB-based blockchain data cache for light nodes (#251)
  Update libp2p again (#445)
  Update version on git head change (#444)
  Fix the public key of bootnode 3 (#441)
  Update libp2p (#442)
  Switch to the master branch of libp2p (#427)
  Export ws port 9944 and add doc (#440)
  Iterate over overlay to decide which keys to purge (#436)
  Exit signal gets its own trait (#433)
  Add docker image (#375)
  Reset peers.json if the content is not loadable (#405)
  Limit number of incoming connections (#391)
  Fix memory leaks in libp2p (#432)
  Do not queue empty blocks set for import (#431)
  5 random fixes (#1) (#435)
  Chore: fix typo (#434)
  Prevent building invalid blocks (#430)
  Better logging for public key mismatch (#429)
  ...