Slow performance caused by `get_remote_ip_and_port` and `get_local_ip_and_port`

[yhirose]

Currently, one of bottle necks of performance is caused by get_remote_ip_and_port and get_local_ip_and_port.

cpp-httplib/httplib.h

Lines 6961 to 6967 in 61c4180

	strm.get_remote_ip_and_port(req.remote_addr, req.remote_port);
	req.set_header("REMOTE_ADDR", req.remote_addr);
	req.set_header("REMOTE_PORT", std::to_string(req.remote_port));
	strm.get_local_ip_and_port(req.local_addr, req.local_port);
	req.set_header("LOCAL_ADDR", req.local_addr);
	req.set_header("LOCAL_PORT", std::to_string(req.local_port));

These slow functions are called regardless of whether REMOTE_ADDR, REMOTE_PORT, LOCAL_ADDR, or LOCAL_PORT will be used later on.

I am thinking of making them "lazy evaluation" calls, so that we don't have to pay such costs.

• REMOTE_ADDR, REMOTE_PORT, LOCAL_ADDR, LOCAL_PORT will be removed from Request
• remote_addr, remote_port, local_addr, local_port will be removed from Request
• get_remote_ip, get_remote_port, get_local_ip and get_local_port will be added

This will be a break change, and will be available from v0.18.0.

[yhirose]

To avoid the breaking change, we can share the same information per socket connection. In other words, we can share it over requests in the same KeepAlive connection.