bsc_subscriber: Optimize lookup of bsub by TMSI

It was found that on a busy osmo-bsc process (>1000 concurrent calls
spead over different BTSs), a good amount of time is spent iterating the
subscribers list trying to find a subscriber based on a TMSI (1.60% of
total CPU time used by osmo-bsc).

This patch introduces a new rbtree under struct bsc_subscr_store which
allows storing all the busbs ordered by TMSI.
This way, lookup time changes O(N) -> O(log(N)), at the expense of
increased insert/deletion time O(1) -> O(log(N)).

Related: SYS#6200
Change-Id: If27429e715ef4b327177e427249e68321a6e83cc
---
M include/osmocom/bsc/bsc_subscriber.h
M src/osmo-bsc/bsc_subscriber.c
M src/osmo-bsc/osmo_bsc_bssap.c
M tests/subscr/bsc_subscr_test.c
4 files changed, 104 insertions(+), 6 deletions(-)