MINOR: quic: support sbuf allocation in quic_stream

MINOR: quic: support sbuf allocation in quic_stream

This patch extends qc_stream_desc API to be able to allocate small
buffers. QUIC MUX API is similarly updated as ultimatly each application
protocol is responsible to choose between a default or a smaller buffer.

Internally, the type of allocated buffer is remembered via qc_stream_buf
instance. This is mandatory to ensure that the buffer is released in the
correct pool, in particular as small and standard buffers can be
configured with the same size.

This commit is purely an API change. For the moment, small buffers are
not used. This will changed in a dedicated patch.