CLEANUP: quic: drop the name "conn_stream" from the pool variable names

CLEANUP: quic: drop the name "conn_stream" from the pool variable names

QUIC was the last user of entities with "conn_stream" in their names,
though there's no more reason for this given that the pool names were
already pretty straightforward. The renaming does this:

  qc_stream_desc: pool_head_quic_conn_stream -> pool_head_quic_stream_desc
  qc_stream_buf:  pool_head_quic_conn_stream_buf -> pool_head_quic_stream_buf