simplest NIO server to have a sense of how fast It can be
improve performance

1. TCP buffer most of time is empty, writable(8K ~ 16K). just write without asking for writable, save 2 system call when data is less than buffer size.
2. IO model changed: One thread doing IO => 1 thread reading and decoding + many thread writing.
3. On I72600 102289.24 => 193963.60
clean code