A almost rewrite of #12537

Prototype of #11725

Implementations

• buffered_io_socket_handle_impl
  Replaced the BufferSource transport socket. So existing transport socket could be supported. Special thank to @florincoras 's work dropping the file descriptor in transport sockets and other filters!
  This is an socket handle which owns a buffer. Think the buffer as the socket buffer in the kernel. This handle should be always created with a peer so that this handle could write to peer's buffer, and notify readable or writable signal along with read/write/shutdown.
• new api in dispatcher
  Allow register internal listener and target listener when a client connection need to be created
• new internal listener impl along with connection_handler
  Register internal listener by name and "accept" connection. Currently internal listener could simulate tcp listener but not udp listener.

Past work

• Address
  Envoy Internal address type is introduced in api: add envoy internal address #12837A new type of address, or specialize the ip/pipe?
• version 1 of the implementation
  [WIP] Envoy Internal connection strawman #12537 Dropped since I force pushed...