fix cwnd compute

https://github.com/skywind3000/kcp/blob/cbf46fa1e94adce82df337d0c4351…

…f164ba327fa/ikcp.c#L885-L889

remove variant cipher block size, cause variant blocksize can cap to

8/16 bytes

fixed a bug on ack.sn math

use absolute time

prevent slice from holding reference to closure

optimize timedsched memory allocation

optimize timedsched

add a library level timedsched.go

Squashed commit of the following:

commit 2553f5d
Author: xtaci <[email protected]>
Date:   Fri Jan 3 19:49:16 2020 +0800

    update xorsimd

commit 0060649
Author: xtaci <[email protected]>
Date:   Fri Jan 3 19:43:28 2020 +0800

    upgrade xorsimd