Remove conn and user_data from encrypt/decrypt/hp_mask callbacks so
that they can be used without conn.

Bring back the processing of Handshake packets.  Doing so can reset
idle timer and reset PTO count which is useful to deal with packet
loss during handshake.

With this change, ngtcp2 library does not increase remote stream limit
automatically.  Application should extend the stream limit by calling
ngtcp2_conn_extend_max_streams_bidi or
ngtcp2_conn_extend_max_streams_uni.  Automatic increment is
problematic because the application stream might be closed well after
QUIC stream is closed (e.g., lost packets for QPACK stream might delay
HTTP level stream closure).

The rule is that if ngtcp2_stream_open is called, then application is
responsible to extend the stream limit.

Remove redundant Buffer::begin

Stream must be closed in nghttp3 callback because quic stream close
comes before http stream close because of packet reordering or loss.

It always equals to buf.data().

Client doesn't read Short packet until handshake completes, so doing
these stuff in one place after handshake is much simpler.

Fix setting remote transport params order

Now server can perform path validation without chaining DCID if a
remote peer does not change DCID as per spec.

Fixes: https://github.com/ngtcp2/ngtcp2/issues/213

Fix typo

Time = 0 is valid time in ngtcp2

so that application is not bothered by another cryptic function.