Remove related flows together with PFCP sessions

[ivan4th]

This is needed to avoid losing IPFIX reports for the flows that still exist when a PFCP session is deleted. As part of the change, flowtable hooks are replaced with flowtable events which are more flexible (multiple handlers per event). Possible inconsistencies related to flows that don't correspond to any active session are now being avoided.

[ivan4th]

A bit of explanation:

• there was a “hook” mechanism that enabled to lessen circular dependencies between the flowtable code and the rest of UPG; the problem with it was that it only allowed one handler per hook, which became inconvenient with the new changes. It has been replaced with 'events', with some added comments: https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/flowtable.h#L237-L267
• Sessions' flows are organized in a singly-linked list. Each flow points to the next one belonging to the same session: https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/flowtable.h#L168 and the session contains the index of the first flow: https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/upf.h#L800
• when a flow is allocated, it's linked into the chain by making it point to the previously-first flow for the session, and then making session's first flow index point to it: https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/upf_flow_node.c#L225-L237
• when a flow is deleted, first FLOW_EVENT_REMOVE occurs which is handled by the IPFIX code to send the final report, if needed https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/upf_ipfix.c#L492 and also by the proxy https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/upf_proxy.c#L1403 so as to remove any connections related to this flow. There's some complication with half-open active-side connections, when such one is present for the flow, the proxy session is unlinked from the flow and kept till the callback is invoked, which will then proceed with further cleanup. This event is also handled in upf.c to update the flow counters https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/upf.c#L501-L522
• after all FLOW_EVENT_REMOVE handlers are called, FLOW_EVENT_UNLINK is invoked which is handled here https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/upf_pfcp.c#L1169-L1208 Basically, it removes the flow from the linked list of the flows belonging to the particular session. There's a bit of compromise in this part: removing the flow costs O(n), n being the number of flows per session. We could use a doubly-linked list instead, so as to have O(1) during flow deletion, but that would further enlarge the flowtable entries, decreasing performance. (Alternative, maybe implement later: keep flow list linkage as a separate vector) The
• in pfcp_free_session, all of the flows belonging to it are forcibly removed: https://github.com/travelping/upg-vpp/blob/56b2959241ebd859d5cf07c77f4c5f8180f2e539/upf/upf_pfcp.c#L1140-L1151 This will invoke FLOW_EVENT_REMOVE and FLOW_EVENT_UNLINK events for each flow, making sure IPFIX reports are sent and proxy connections are dealt with.