Bunch of changes related to block log

Related: hive!731 (merged), haf!179 (merged)

Changes:

• Block log index was replaced with block log artifacts.
  • No more following warnings at CI: UserWarning: Block log with following path is missing: InitNode0/blockchain/block_log.index.
• Block log artifacts during replay are treated as optional.
  • Previously if were missing, warning was generated, and flag include_artifacts=False had to be used.
  • Now artifacts are used if provided. If are missing, then everything is OK. Replay is performed without them.
• BlockLog.copy_to now returns copied block log object.
• During copying block log, artifacts can be treated as required, optional or excluded.
  • Are excluded by default, to simplify use case, where user generates block log for tests and copy it to predefined location.
  • Removed specifying how artifacts are treated during copying from block log's initializer (because it's unneeded, way of copying artifacts is specified during copying with BlockLog.copy_to artifacts parameter).
  • Valid values of artifacts parameter are suggested with type annotations and enforced in runtime.
• Following members are now @properties:
  • Block log can be get from node with syntax node.block_log (previously was node.get_block_log()).
  • Path of block log can be get with syntax block_log.path (previously was block_log.get_path()).
  • So now, to get path of node's block log syntax was simplified from node.get_block_log().get_path() to node.block_log.path.
• Fix bug, which caused error when node was replayed from renamed block log.
• Fix bug, which caused that name of requested output block log was ignored during block log truncation (in BlockLog.truncate method).
• Add documentation of BlockLog.truncate method.
• Add block log artifacts path getter (with @property, consistently with path of block log).
• Prevent copying block log files partially when error occurs (previously artifacts could be copied, when block log was missing).

Minor changes:

• "Block log" (instead of "blocklog", as single word) is consistenly written in whole project.
• Member variable __owner was removed from BlockLog.
  • So owner do not have to be passed in initializer. It simplifies block log creation (from BlockLog(None, path) to BlockLog(path)).
• Fixed error message for missing artifacts in given path (previous error message was about block log, not artifacts).
• Add type annotations for path parameters (Union[Path, str]).
• Minor refactors:
  • Method Snapshot.copy_to was simplified with helper method __copy_file.
  • Remove unneeded local variable block_log_artifacts_file_name.

Please review: @kmochocki @mzebrak