* Xtls: code refactor
- Move more logic to VisionReader/Writer
- Remove XtlsWrite()
- XtlsRead now only handle splice at the outbound
- This helps VLESS inbound to have simple buf.copy() so that we can remove pipe next
* Add bufferFlushNext; Use DispatchLink() in VLESS inbound
* Use TimeoutWrapperReader; clean up timer/buffer