- [FEATURE] QUIC and HTTP/3 Internet Draft 29 support.
- [BUGFIX] Check that scheduled packets are also sendable when
  calculating a connection's "tickable" property.
- [BUGFIX] Don't count scheduled packets as in-flight when pacer is
  checked on tick.
- gQUIC: delay calling on_new for pushed stream until headers are
  available.
- Allow nested calls to lsquic_engine_connect().