Rewrite ProxymanCore with Swift NIO

Description

ProxymanCore v1 is built on top of CocoaAsyncSocket, which has some major problems:

• Don't Support HTTP/2, we have to manually implement it, which is error-prone
• Not an HTTP Server, so we have to deal with all HTTP Message Parsers
• CocoaAsyncSocket doesn't seem active anymore. We encountered random crashes from deep inside the socket with no reason. (Ex: Crash in SSL_Handshake, closeSocket, ...)
• ProxymanCore v1 architecture is not good for extending for future features, such as SOCK Proxy, ReWrite, gRPC...
• Poor performance in some cases since CocoaAsycnSocket doesn't tend for high-performance network tools with multiple incoming/outcoming requests, response

It's time to move to Swift NIO, which is a cross-platform asynchronous event-driven network application framework for rapid development of maintainable high-performance protocol servers & clients.
Other benefits would be:

• Built-in HTTP Server
• Officially support HTTP/2
• Has gRPC 3rd-party
• Use BoringSSL so we can get rid of spawn a openssl to generate certificates => Suitable for iOS app
• Highly reusable on iOS and Linux (Window is possible in the near future)
• Battle-testing and active development from Apple Team 🙌
  ......
  Examples:
• HTTP/2: https://github.com/apple/swift-nio-http2
• SSL: https://github.com/apple/swift-nio-ssl
• Proxy: https://github.com/apple/swift-nio-examples/tree/master/connect-proxy