Add load regulation based on packet stats and delay in port msg queue

LINC-Switch was getting overloaded under high packet rates. This commit
introduce mechanism that allow LINC to detect such situations
and prevent overload. This mechanism is based on how many packets
goes through the switch.

If a particular port is overloaded it stops transmitting packets,
receiving packets or even both.

Motivated by #301.
Add tests to load regulation

Motivated by #301.