BUG/MEDIUM: quic: free stream_desc on all data acked

BUG/MEDIUM: quic: free stream_desc on all data acked

The following patch simplifies qc_stream_desc_ack(). The qc_stream_desc
instance is not freed anymore, even if all data were acknowledged. As
implies by the commit message, the caller is responsible to perform this
cleaning operation.
  f4a83fbb14bdd14ed94752a2280a2f40c1b690d2
  MINOR: quic: do not remove qc_stream_desc automatically on ACK handling

However, despite the commit instruction, qc_stream_desc_free()
invokation was not moved in the caller. This commit fixes this by adding
it after stream ACK handling. This is performed only when a transfer is
completed : all data is acknowledged and qc_stream_desc has been
released by its MUX stream instance counterpart.

This bug may cause a significant increase in memory usage when dealing
with long running connection. However, there is no memory leak, as every
qc_stream_desc attached to a connection are finally freed when quic_conn
instance is released.

This must be backported up to 3.1.