Changes

library/PFCP_Emulation: a better PDU routing concept

In recently merged 2962d170 I wrongly assumed, that SEID of outgoing
PFCP PDUs can be used to correlate and route the incoming PDUs.  In
fact, the PFCP peers use two different SEID values, negotiating them
using the F-SEID IE.

We could have implemented a logic to look for F-SEID in the outgoing
PDUs, store and then use it for routing.  However, a more flexible
approach is to allow the the PFCP_ConnHdlr components to subscribe
and unsubscribe to/from specific SEID values explicitly.

In this spirit, let's allow the PFCP_ConnHdlr components to subscribe
and unsubscribe to/from broadcast PDUs (i.e. those, for which the
PFCPEM component could not find a single recipient) explicitly.

Implicit routing using the SeqNr remains unchanged and will be
performed by the PFCPEM component automatically like before.

Change-Id: I25802471519fa297ad4cb2b056adaa6748b00af2
Related: 2962d170 "library/PFCP_Emulation: fix routing of incoming PDUs"