HTTP/[23]: continue upload when state.drain is set

HTTP/[23]: continue upload when state.drain is set

- as reported in #10433, HTTP/2 uploads may stall when a response is
  received before the upload is done. This happens when the
  data->state.drain is set for such a transfer, as the special handling
  in transfer.c from then on only cared about downloads.
- add continuation of uploads, if applicable, in this case.
- add pytest case test_07_12_upload_seq_large to reproduce this scenario
  (although, current nghttp2 implementation is using drain less often)

Reported-by: Lucas Pardue
Fixes #10433
Closes #10443