Improvement: Log operational events

[mgumz]

Operational events must be logged so they become visible for operations. Such events are not happening in the 1000/s but only once in a while and should not impose any noticeable load onto upg-vpp when logging. Some of such events:

• PFCP Association Established with a SMC (the node name of SMC in addition to the IP)
• PFCP Association Disconnected (from which SMC, IP)
• PFCP Association becomes suspicious (and upg-vpp starts resending Heartbeat-Requests)
• PFCP Association "Lost"
• NAT-Pool saturation / exhaustion level reached

[mgumz]

related: 144.

[korroot]

Closed by: #294