MINOR: h3: mark control stream as metadata

MINOR: h3: mark control stream as metadata

A current work is performed to change QUIC MUX buffer allocation limit
from a configurable static value to use the size of the congestion
window instead. This change may cause the buffer allocation limit to be
triggered more frequently.

To ensure HTTP/3 control emission is not perturbed by this change, mark
the stream with qcc_send_metadata(). This ensures that buffer allocation
for this stream won't be subject to the connection limit. This is
necessary to guarantee that SETTINGS and GOAWAY frames are emitted.