Changes

s1ap_proxy: erab_fsm_start_reg/2: check if already registered

Before this patch, s1ap_proxy would spawn a new erab_fsm process with
a duplicate UID and simply replace the old process in the rigistry.
This would result in stale erab_fsm process, still linked but not
present in the registry, and thus not reachable.

If there's already an E-RAB process for the given UID, simply return
Pid of that existing process.  The FSM will most likely reject an
unexpected event, resulting in a negative verdict:

* 'reply' for duplicate E-RAB SETUP REQUEST
* 'drop' for duplicate INITIAL CONTEXT SETUP REQUEST

Change-Id: I8e93492219100496fd8dc69e031310858ba9c7fb