Cygwin: ST stuck when working in multiple threads mode.

[winlinvip]

Workaround: Force to use single thread by --single-thread=on as a workaround.

We must use single thread for cygwin64, because there is a bug while publishing is timeout after about 25s and the RTMP receiving coroutine not working correctly. Need more time to finger out why, so we just use single thread as workaround.

[winlinvip]

The root cause for Windows SRT crashing is not about threading but C++ exception SEH, see #3251 for details, so we need to revert this limit and enable multiple threading for windows, again without SRT. This still needs more research and testing.