This commit adds special handling for the very first Initial packet
which is corrupted and discarded.  If server receives such packet, it
has to drop connection silently without sending any CONNECTION_CLOSE.
Then valid packet might come later.  Keeping connection state is
harmful because corrupted packet has wrong SCID, which drops later
valid packet because mismatched SCID.  New error code
NGTCP2_ERR_DROP_CONN is added which tells server application to drop
connection silently.

Require token if the first packet is lost or reordered in order to
buffer the data coming from validated client only.

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.