!str: #15474: Migrate to reactive-streams 0.4.0.M1
Merge pull request #15578 from drewhk/wip-15474-streams-migrate-spi-0.4-drewhk

Migrate streams to reactive-streams 0.4.M1
=htc #15560 close entity data stream eagerly + add more tests
=htc fix off-by-one error when creating strict entities for small payloads