Posix net.hpp refactor for reference

[MarkReedZ]

My thought for the refactor was to create a net.hpp ( later net-uring.hpp / net-posix.hpp ) to handle the networking using a common engine.cpp. This is a prototype for the posix engine only. Not expected to be merged though tests pass and python works though I didn't add python tests.

net_init is called passing an on_data function.

  fd = net_init(config, on_data);

 int on_data(conn_t *conn, char *buf, int data_left) {
    engine_t& engine = *reinterpret_cast<engine_t*>(conn->user_data);

Interesting things:

• Posix now handles large requests > 4096 and partial requests. In the on_data function you return the data left after parsing and the next on_data call will contain the remaining + new data.
• I use a map for the callbacks. I updated engine-uring.cpp with this so you can see it below
• We detect json by looking at the first character and handle json and http separately
• take_call is not supported so python uses take_calls