Changes

s1gw: move PFCP association handling into a dedicated ConnHdlr

Previously, the PFCP association request from the IUT was handled by
the first ConnHdlr component (idx := 0).  While this approach has
worked, it fails when multiple ConnHdlr instances (idx > 0) are spawned.

The problem arises when other ConnHdlr (idx > 0) instances initiate
PFCP procedures before the first ConnHdlr (idx := 0) has established
the association, so we end up playing races.

This patch introduces a dedicated ConnHdlr component to handle the
PFCP association independently.  Once the association is established,
the actual test ConnHdlr instances are spawned, ensuring a more
reliable and orderly process.

Change-Id: I5e1a14105a35b785bf598dc2a8c436ea6fb6b0f1