http2: improved upload eos handling

http2: improved upload eos handling

- replace the counting of upload lengths with the new eos send flag
- improve frequency of stream draining to happen less on events where it
  is not needed
- this PR is based on #14220

http2, cf-h2-proxy: fix EAGAINed out buffer
- in adjust pollset and shutdown handling, a non-empty `ctx->outbufq`
  must trigger send polling, irregardless of http/2 flow control
- in http2, fix retry handling of blocked GOAWAY frame

test case improvement:
- let client 'upload-pausing' handle http versions

Closes #14253