Optimize subscr_conns lookup

It was found that, on a busy osmo-bsc (>1000 concurrent calls spread over
several BTS), the CPU consumption looking up for gsm_subscriber_conn
based on SCCP conn_id can take a considerable amount of time (>5% of
osmo-bsc already taking 70% of the CPU time on the core it is running on).

The huge CPU consumption happens because a linear search is done (llist)
over the entire list of SCCP connections every time an SCCP message is
handled.

In order to optimize this lookup, this patch introduces a new struct
bsc_sccp_inst which becomes associated to the libosmo-sccp
osmo_sccp_instance. Each of this strucs maintains an rbtree of
gsm_subscriber_conn ordered by conn_id.
As a result algorithmic complexity adds O(log(N)) during insert, lookup
and delete of SCCP conns, but gets rid of O(N) previous lookup.
As a plus, finding a new conn_id now takes generally O(log(N)), while
before it used to take O(N).

Related: SYS#6200
Change-Id: I667d3ec1dad0ab7bc0fa4799d9611f3a914d07e5
---
M TODO-RELEASE
M include/osmocom/bsc/gsm_data.h
M src/osmo-bsc/bsc_sccp.c
M src/osmo-bsc/bsc_subscr_conn_fsm.c
M src/osmo-bsc/lb.c
M src/osmo-bsc/osmo_bsc_sigtran.c
6 files changed, 183 insertions(+), 101 deletions(-)