Add header only support for core implementation(c++)

Sync v3.22.1 finally finally (#116)

* Update README.md

* Optimize sources.

* Rename io_service's member transports_dypool_ to transports_pool_

* Tidy sources.

* Update description.

* Fix compile issues for HEADER ONLY support.

* Fix compile issues with YASIO_HAVE_KCP for HEADER ONLY support.

* Sync kcptest.cpp

* Remove unused definations

* Optimize includes for YASIO_HEADER_ONLY

Use inline namespace avoid conflicit on Apple platform.