#146: Use f-string

• Fix local blockchain code
• Delete empty api.elasticsearch module
• Use f-string as much as possible:
  • Replace .format(), %, str + str formats to f-string format
  • Done as much as possible without putting too horrible operations in the f-string
  • Kept % format into the logging for now, since it causes errors with pylint and mypy

Close #146 (closed).