BUG/MINOR: hq-interop: simplify parser requirement

BUG/MINOR: hq-interop: simplify parser requirement

hq-interop should be limited for QUIC testing. As such, its code should
be kept plain simple and not implement too many things.

This patch fixes issues which may cause rare QUIC interop failures :
- remove some unneeded BUG_ON() as parser should not be too strict
- remove support of partial message parsing
- ensure buffer data does not wrap as it was not properly handled. In
  any case, this should never happen as only a single message will be
  stored for each qcs buffer.

This should be backported up to 2.6.