AS loadsharing: Initial routing implementation based on extended-SLS

This commit adds a new set of APIs and structs to support routing based
on OPC,DPC,SLS fields and hence to support AS-loadshare.

The routing decision is still actually left as beforehand, ie. the first
AS/linkset in the combined_linkset is selected.

This will allow future work, i.e to:
    - Check if data can be sent over the combined link (at least one
      AS/linkset is ACTIVE), and otherwise try using a combined link with a
      less specific match (shorter prefix mask or/and lower priority).
    - Select one of the AS/linksets in the combined link based on a
      round robin approach.

Related: SYS#7112
Change-Id: I0fb4ca4959096f748a23082efa0481300de56436
---
M include/osmocom/sigtran/osmo_ss7.h
M src/osmo_ss7_combined_linkset.c
M src/osmo_ss7_hmrt.c
M src/osmo_ss7_instance.c
M src/osmo_ss7_route.c
M src/osmo_ss7_route_table.c
M src/osmo_ss7_vty.c
M src/sccp_scrc.c
M src/ss7_combined_linkset.h
M src/ss7_instance.h
M src/ss7_route.h
M src/ss7_route_table.h
M src/xua_rkm.c
M src/xua_snm.c
M tests/ss7/ss7_test.c
M tests/vty/osmo_stp_test.vty
16 files changed, 280 insertions(+), 42 deletions(-)