About the new redesigned I/O Multiplexing backend

[halx99]

yasio::inet::io_primitive:

• iocp_io_primitive for windows
• epoll_io_primitive for linux
• kqueue_io_primitive for bsd(macOS,ios,tvOS)
• poll_io_primitive for all os
• select_io_primitive for all os
• evport_io_primitive for solaris

basic types:

yasio::inet::io_operation (OVERLAPPED or reactor non-blocking io)

APIs of io_primitive

• do_open(xxsocket& s)
• int connect(io_base*, const xxsocket& s, const ip::endpoint&)
• int accept(io_base*)
• int send(io_base*)
• int recv(io_base*)
• int poll_io(int64_t wait_us)
• void wakeup()