During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
Part of #9744
Removes all redundant `# -*- coding: utf-8 -*-` lines from files, as python 3 automatically reads source code as utf-8 now.
`Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>`
Handle the fact that events can be persisted out of order, and so to get
the "current max" stream token becomes non trivial - as we need to make
sure that *all* stream tokens less than the current max have also
successfully been persisted.
