Extracted from #20186 (branch rei/se_sharded_tok at 8e10c4def4) so that
receipt paging for application services can use them without waiting
for the sticky events changes in that PR.
This is that branch's net change to these files relative to develop:
the helpers in id_generators.py for bounding a query on a multi-writer
stream by sharded tokens and for advancing a sharded token after a
limited read, their tests, and the test assertion helpers those tests
use.
Olivier 'reivilibre
a8b9fa84ee