better performance by io.copy. zero copy transfer

[cppla]

hi, big guy. zero copy by splice function and sendfile function，better performance for big file and more and more little file

Write and Read

- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-60.00  sec  27.5 GBytes  3.94 Gbits/sec  196             sender
[  5]   0.00-60.00  sec  27.5 GBytes  3.94 Gbits/sec                  receiver

CPU: 50%-%65%, max:80%

io.copy

- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-60.00  sec  28.0 GBytes  4.00 Gbits/sec  224             sender
[  5]   0.00-60.00  sec  27.9 GBytes  4.00 Gbits/sec                  receiver

CPU: 10%-%12%, only 10% usage

[cppla]

Zero Copy is a greate ways.

good luck .

[crabkun]

Nice PR. Thanks!!