Support Multiple Threads for Linux and Darwin only.

[winlinvip]

For WebRTC or UDP transport system, multiple threads or CPUs is essential important, please read ossrs/srs#2188

The first step, is to simplify the state-threads. We should remove the dead code for UDP server:

• Remove multiple OS support, only for Linux(CentOS,Ubuntu,etc) and Darwin(macOS).
• Remove the examples and extensions.
• Remove the poll support, only Linux epoll and Darwin kqueue.
• Remove the support for multiple processes, for single process only.
• Stack always grows from top to down.
• Remove the deprecated serialize accept.

Then, we should use gcc __thread for multiple threads:

• Support gtest and gocv/gcovr coverage analysis.
• Change all global or static variables to __thread, see toffaletti befca4dfbd, 9b617a6a02, f0ebf18d8f, 527931ce8e.
• Add utest for threads and codes. #31

对ST做了大幅度简化：eda23b2

For #19: Simplify ST, only support LINUX and DARWIN

1. Remove examples and extensions.
2. Remove OS support, except LINUX and DARWIN.
3. Rename md.S to md_linux.S, remove __ia64__ support.
4. Remove events support, except kqueue and epoll.
5. Refine utest, add auto/fast.sh support.

简化前，一共是9123行，核心代码5834行：

homeMacOS:state-threads winlin$ wc -l *.h *.c *.S 
     480 common.h
     645 md.h
     176 public.h
    1446 event.c
     922 io.c
     121 key.c
     783 sched.c
     173 stk.c
     368 sync.c
     644 md.S
      76 md_darwin.S
    5834 total

简化后，核心代码4448行：

homeMacOS:state-threads winlin$ wc -l *.h *.c *.S 
     473 common.h
     232 md.h
     174 public.h
     913 event.c
     815 io.c
     121 key.c
     751 sched.c
     173 stk.c
     368 sync.c
      76 md_darwin.S
     352 md_linux.S
    4448 total

一共简化了1386行代码，23%左右代码。简化后，UTest和覆盖的难度降低了，之前存在大量无法覆盖的代码。

简化后，只支持LINUX和DARWIN平台，事件框架只支持kqueue和epoll。

[winlinvip]

UTest and coverage for ST is introduced by SRS#1b2822a5a4, of course, it'll be merged to this repository if done.

[winlinvip]

#19

[winlinvip]

ST: Support thread-local for multiple threads.