add support for xchacha20-ietf-poly1305 set nonce to 24 for ChaCha20P…

[lielongxingkong]

add support for xchacha20-ietf-poly1305 set nonce to 24 for ChaCha20Poly1305 in botan-2

[imknown]

Updated: 2018-12-11 09-42-00

It works after using Botan 2.8.0.

https://github.com/randombit/botan/blob/master/news.rst#version-280-2018-10-01

But too many logs printed and too laggy when network is not decent:

AEAD data chunk is incomplete (too small for payload)
AEAD data chunk is incomplete (too small for length)

Same here: #165 , #177 , https://github.com/shadowsocks/shadowsocks-qt5/issues/737

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Origin I built with your PR, but it crashes:

➜  build git:(master) ss-qt5
Initialising cipher: xchacha20-ietf-poly1305
Running in local mode.
Listen UDP socket state changed to QAbstractSocket::BoundState
TCP server listening at 127.0.0.1:1081
Connecting www.google.com:443 from 127.0.0.1:46818
terminate called after throwing an instance of 'Botan::Invalid_IV_Length'
  what():  Invalid argument IV length 24 is invalid for ChaCha20Poly1305
[1]    7306 abort (core dumped)  ss-qt5