Currently, there's no upper bound for how many requests that ctrld will
handle at a time. This could be problem on some low capacity routers,
where CPU/RAM is very limited.
This commit adds a configuration to limit how many requests that will be
handled concurrently. The default is 256, which should works well for
most routers (the default concurrent requests of dnsmasq is 150).
This commit add the ability for ctrld to gather client information,
including mac/ip/hostname, and send to Control-D server through a
config per upstream.
- Add send_client_info upstream config.
- Read/Watch dnsmasq leases files on supported platforms.
- Add corresponding client info to DoH query header
All of these only apply for Control-D upstream, though.
