Use connection id when allocating rate counters

Due to recent libosmocore's change we can't allocate rate counters with
the same name and index which are already allocated. This causes
sgsn_mm_ctx_alloc_iu() failure for multiple subscribers.

Fix this by using conn_id parameter from ranap_ue_conn_ctx.

Change-Id: I1062ffdcac96c82269cab6f4e7ae50e28dc3aa44
Related: OS#2757