Resolve "Protect metadata gathering from parallel scans"

Closes #2185 (closed)

Introduction of ICAT metadata and the Dataset in Redis caused 2 race conditions for parallel scans:

• ESRFScanSaving.on_scan_run called in multiple greenlets on a clone of the same scan saving:
  • issue: it raised on exception that the metadata was already gathered
  • solution: dataset.gather_metadata(on_exists="skip") no longer raises an exception when the metadata is already gathered. Calling dataset.gather_metadata() still raises an exception when the metadata is already gathered (and it should).
• get_node of a Dataset in Redis would sometimes return a DataNodeContainer:
  • issue: get_node would return a DataNode even when it is not fully instantiated yet (in this case the node_type is not set yet)
  • solution: the principal Redis key of the DataNode now has a version property (str) which is set after DataNode.__init__ is completed with create=True and before adding itself to its parent.
• as a side effect: nodes with unsupported versions in Redis are ignored (by default) by the utility functions get_node, get_nodes, etc.

Note that the last change doesn't require a db=1 flush. get_node will return None if the node's version doesn't match the current implementation (before it returned None when the name property was None; which meant the key didn't exist or was in the process of being created). So Flint and the writer will not accept nodes that are not supported.