ctrl, hnbgw: access rate counter groups by given ID instead of index

Teach Osmocom_CTRL_Functions to retrieve rate counters by the ID that
the program has set with rate_ctr_group_set_name() (e.g. a hNodeB's cell
id).

In our tests, the logic is firmly built on indexed arrays. For example,
for CN pooling, the tests conveniently have an array of msc.0, msc.1
etc.. This clashes with objects where the indexes on the CTRL interface
may vary dynamically. This patch allows the same indexed handling of
rate counters even when using the ID names instead of the indexes: still
keep the objects in an indexed array, and also provide a mapping from
index to ID name.

Specific motivation:

Since osmo-hnbgw commit 61e278a452bf4fd240e45f1fe8c094a4b3795317
"hnb_persistent: Use incrementing counter for rate_ctr + stat_item index"
the hnb rate counter index increments after a hnbp has expired.

Instead, use the hNodeB's cell ID (which osmo-hnbgw sets on the counter
group with rate_ctr_group_set_name()) to retrieve counters from the CTRL
interface.

Depends: libosmocore I841a12f76e6fcb2bd7aecb5f4b1707d9ec927137
Related: OS#6545
Change-Id: I70e74e7554482df67aa1d90bc04314124dea444f
---
M hnbgw/HNBGW_Tests.ttcn
M library/Osmocom_CTRL_Functions.ttcn
2 files changed, 55 insertions(+), 22 deletions(-)