synchronized batch sending

v5.3.5

do not close socket in synchronized sending

v5.3.4

fix

proper error codes

v5.3.2

proper error handling

sendmmsg

1. sendmmsg
2. better error handling
3. for Read() data will be drained before receiving error.

v5.2.8: Xtaci patch 1 (#119)

* Add files via upload

* Update README.md

v5.2.7

move batchSize to readloop_linux.go

conditional build for linux and generic

v5.2.6

conditional build for linux and others

optimize with recvmmsg for linux system

recvmmsg will be much faster than recvfrom for incoming udp packets, but only available on linux

v5.2.4

omit the data copying before headerOffset

v5.2.3

fix time comparsion on edge