NGTCP2_ERR_CONGESTION was removed.  NGTCP2_ERR_NOBUF won't be returned
anymore from ngtcp2_conn_write_handshake, ngtcp2_conn_write_pkt, and
ngtcp2_conn_writev_stream (and their family functions).  They return 0
when previously they return NGTCP2_ERR_CONGESTION or NGTCP2_ERR_NOBUF.

NGTCP2_MIN_PKTLEN was removed.  Now it tries writing a packet to a
buffer limited by CWND, and falling back to ACK only packet if it
cannot write anything.