RFC: Introduce support for MGW-pinning per BTS

This feature allows pinning each BTS to a specific MGW from the
configured pool. The pinning can be soft or hard (strict). If strict
pinning is not set, the configured MGW is selected with priority, but
other MGWs can still be selected during each call setup if the
preferred MGW is found not available at that time, hence avoiding denial
of service for the entire BTS if that MGW goes down.
If strict mode is selected, the call will be refused if the configured
preferred MGW is not available at the time the call is set up.

It is useful to use this feature when Osmux is configured between
the BTS and the BSC and an MGW pool is in use. This way the BSC is
capable of grouping all the calls of a BTS towards one MGW, hence taking
advantage of the Osmux trunking optimizations to reduce link data usage
(AMR payload of several concurrent calls ending up sharing the same
underlaying UPD packet).
Furthermore, this allows the operator to intelligently spread load over
the MGW pool in order to avoid ending up with more than 256 concurrent
Osmux circuits on any of the co-located MGWs in the pool (maximum supported
at the moment).

Related: SYS#5987
Change-Id: I9a7a5af72795faed0d12d9d73b59951b6a0e9c7d
---
M doc/manuals/chapters/mgwpool.adoc
M include/osmocom/bsc/bts.h
M src/osmo-bsc/bsc_subscr_conn_fsm.c
M src/osmo-bsc/bts.c
M src/osmo-bsc/bts_vty.c
5 files changed, 152 insertions(+), 1 deletion(-)