Add option to configure load balance sockets

Previously, the option to enable kernel load balancing of the sockets was always enabled when supported by the operating system (SO_REUSEPORT on Linux and SO_REUSEPORT_LB on FreeBSD).

It was reported that in scenarios where the networking threads are also responsible for processing long-running tasks (like RPZ processing, CATZ processing or large zone transfers), this could lead to intermitten brownouts for some clients, because the thread assigned by the operating system might be busy. In such scenarious, the overall performance would be better served by threads competing over the sockets because the idle threads can pick up the incoming traffic.

Add new configuration option (load-balance-sockets) to allow enabling or disabling the load balancing of the sockets.

Closes #3249 (closed)