Changes

Introduce iu_rnc FSM

This FSM is similar to the already existing ran_peer_fsm in osmo-msc,
which already had better logic around SCCP and RANAP state handling.
Similarly, osmo-sgsn's struct ranap_iu_rnc maps to osmo-msc's struct ran_peer.

With this FSM we can currently track the RANAP link state towards a given
RNC peer:
* Reject (RANAP Error Indication) all UE-related messages until a RANAP
  RESET from RNC is received first.
* Tear down all subsriber connections whenever the RANAP peer sends us a
  RESET message.
* Tear down all subscriber connections whenever the SCCP link towards
  RNC becomes unavailable.
* Send a RESET towards RNC peer once the SCCP link towrdards it becomes
  available again.

This commit only implements so far the Rx path of the FSM, ie. when
receiving events/messages from a peer over SCCP and pushing them locally
up the stack (RANAP). The Tx side will be implemented in a follow-up
commit, which will allow discarding messages if the lower layers towards
a given RNC are known to be down.

Change-Id: I18b7803500163e78ff6a684095194174b0fb6ee1