MINOR: mux-quic: reorganize flow-control frames emission

MINOR: mux-quic: reorganize flow-control frames emission

Adjust the mechanism for MAX_STREAMS_BIDI emission. When a bidirectional
stream is removed, current flow-control level is checked. If needed, a
MAX_STREAMS_BIDI frame is generated and inserted in a new list in the
QCS instance. The new frames will be emitted at the start of qc_send().

This has no impact on the current MAX_STREAMS_BIDI behavior. However,
this mechanism is more flexible and will allow to implement quickly
MAX_STREAM_DATA/MAX_DATA emission.