gbproxy: Implement scaling of BVC flow control in SGSN pool

When there are multiple SGSNs inside a pool, we need to decide
how much of the per-BVC capacity advertised by the BSS in its
BVC-FLOW-CONTROL we should announce to each of the pool members.

A conservative approach would be to advertise 1/num_sgsn, but
there may also be use case where over-provisioning (announcing more
than an equal share of the capacity) is useful.

Hence, let's introduce "pool bvc-flow-control-ratio <1-100>" in order
to allow the administrator to decide.

Related: OS#4891
Change-Id: Ibe5addf657e7237499ca0205bacfe999ecd1e771
4 files changed