immediately SCCP RLSD on HNB re-register

The RUA side may disconnect
a) gracefully (RUA Disconnect with RANAP IU Release Complete) or
b) by detecting that the HNB is gone / has restarted

For a), the SCCP side waits for a RLSD from the CN that follows the IU
Release Complete.
For b), we so far also wait for a RLSD from CN, which will never come,
and only after a timeout send an SCCP RLSD to the CN.

Instead, for b), immediately send an SCCP RLSD.

To distinguish between the graceful release (a) and the disruptive
release (b), add new state MAP_RUA_ST_DISRUPTED on the RUA side,
and add new event MAP_SCCP_EV_RAN_LINK_LOST for the SCCP side.

Any non-graceful disconnect of RUA enters ST_DISRUPTED.
disrupted_onenter() dispatches EV_RAN_LINK_LOST to SCCP,
and SCCP directly sends RLSD to the CN without timeout.

These changes are also shown in doc/charts/hnbgw_context_map.msc.

Change-Id: I4e78617ad39bb73fe92097c8a1a8069da6a7f6a1
---
M doc/charts/hnbgw_context_map.msc
M include/osmocom/hnbgw/context_map.h
M src/osmo-hnbgw/context_map_rua.c
M src/osmo-hnbgw/context_map_sccp.c
4 files changed, 95 insertions(+), 15 deletions(-)