http3: improvements across backends

http3: improvements across backends

- ngtcp2: using bufq for recv stream data
- internal stream_ctx instead of `struct HTTP` members
  for quiche, ngtcp2 and msh3
- no more QUIC related members in `struct HTTP`
- experimental use of recvmmsg(), disabled by default
  - testing on my old debian box shows no throughput improvements.
  - leaving it in, but disabled, for future revisit
- vquic: common UDP receive code for ngtcp2 and quiche
- vquic: common UDP send code for ngtcp2 and quiche
- added pytest skips for known msh3 failures
- fix unit2601 to survive torture testing
- quiche: using latest `master` from quiche and enabling large download
  tests, now that key change is supported
- fixing test_07_21 where retry handling of starting a stream
  was faulty
- msh3: use bufq for recv buffering headers and data
- msh3: replace fprintf debug logging with LOG_CF where possible
- msh3: force QUIC expire timers on recv/send to have more than
  1 request per second served

Closes #10772