Change API of Http3RequestStreamInboundHandler to better handle FIN

[normanmaurer]

Motivation:

5740c520bbabd490a448c97a4b8f0574115b68f6 introduced a workaround for correctly handle FIN when multiple http3 frames are contained in one QUIC frame. While this worked it was kind of hacky and we should better adjust the API.

Modifications:

• Change Http3RequestStreamInboundHandler API for handling frames and FIN
• Adjust tests.

Result:

Better fix for handling multiple frames

[normanmaurer]

@yawkat WDYT ?

[normanmaurer]

it's a bit annoying that we cant coalesce FullHttpRequest anymore but i guess that's not doable without buffering

yeah... that said I think its most likely not a big deal and the API / implementation seems a bit better this way.

[normanmaurer]

@yawkat btw I am happy if you have any better idea :)

[yawkat]

i don't :)