Simplify remove_tx_stream_data()

The return value from this function is not used.

This prioritizes Handshake probe packet over Short probe if both data
are outstanding.

This is a deviation from draft-27 recovery draft.

This prevents Handshake probe packet from being sent even if Handshake
packet number space has no outstanding data to send.

This is a deviation from draft-27 recovery draft.

This reverts commit f738004f.

This prevents Handshake probe packet from being sent even if Handshake
packet number space has no outstanding data to send.

This is a deviation from draft-27 recovery draft.

Fix coalesced 0-RTT ACK packet with short header packet

examples/server accepts 2 new query parameters.

- u query parameter takes urgency value [0, 7], inclusive (e.g., u=0).

- i query parameter takes 0 or 1, and they present non-incremental and
  incremental respectively (e.g., i=1).  Note that this is not
  sh-boolean.

The server overrides stream priority with these parameters.

This reverts commit 1e2c09f8.

examples/server accepts 2 new query parameters.

- u query parameter takes urgency value [0, 7], inclusive (e.g., u=0).

- i query parameter takes 0 or 1, and they present non-incremental and
  incremental respectively (e.g., i=1).  Note that this is not
  sh-boolean.

The server overrides stream priority with these parameters.

Clarify the behaviour of ngtcp2_conn_writev_stream with
NGTCP2_WRITE_STREAM_FLAG_MORE.

Round up cwnd left to the maximum UDP packet size to avoid small
packet.

Reset pto_count only when ACK-eliciting, non pure probe packet is
received in order to avoid frequent PTO probe transmission.

This commit separates key installation function into 2 and let TLS
stack install rx and tx keys separately for handshake and 1RTT keys.
This change is aligned to the new BoringSSL API and GnuTLS API.  I
expect that OpenSSL will follow this change.  It also removes side
argument from crypto API if it can be inferred by
ngtcp2_conn_is_server.

crypto: fix a couple of typos in the documentation

crypto: add GnuTLS backend