Changes

ranap: Create iu_rnc upon rx RESET

RANAP RESET, the first message an SGSN usually receives from an RNC,
contain the GlobalRNC-ID we can already use to register the iu_rnc
object.
This allows keeping track of peers since first message, before first
InitialUE message is received.
Once we start tracking peer RNC state within osmo-sgsn (as we already do
in other osmo-* programs), this will be needed.

Change-Id: Iaf31271feb4d88881382ed8594d0d8e20e22b194