Changes

s1gw: f_ConnHdlr_session_delete(): invalidate peer's SEID

Change-Id: If8027e2bdb5822eb7555cd02a72be900a1f18880

s1gw: f_ConnHdlr_session_delete(): respect any order

It's not guranteed (nor required) that PFCP Session Deletion Request
PDUs are sent in the same order as their respective ERab records are
organized in the given ERabList.  They can be emitted in any order.

Make f_ConnHdlr_session_delete() more flexible:

* Expect to receive N PFCP Session Deletion Request PDUs;
* For each received PFCP PDU, find the matching E-RAB;
* Make sure that an E-RAB is never released twice;
* Send PFCP Session Deletion Response.

This patch fixes the following TCs:

* TC_ue_ctx_release_req3
* TC_ue_ctx_release_cmd_compl3

Change-Id: I93d3d38829b1f425d18d59464894da239e3511eb
Related: 75cb01b7 ("s1gw: add TCs for UE CONTEXT RELEASE procedures")
Related: SYS#7310