paging: Replace reqs waiting for retransmission with new incoming inital req if queue is full

If queue size (in transmit delay of requests) is too long (above
threshold) when a new initial incoming request arrives, instead of
directly discarding it, see if we can drop a pending retransmission and
insert the new one instead, in order to avoid losing initial requests.

This is done under the assumption that it is more important to transmit
intial requests than to retransmit already transmitted ones. The
rationale is that there's lower chances that an MS which didn't answer
lately will answer now (aka being reachable at the cell), so it's better
to allocate resources for new requests (new MS) which may be available
in the cell.

Change-Id: Idfd93254ae456b1ee08416e05479488299dd063d
Related: OS#5552
---
M src/osmo-bsc/net_init.c
M src/osmo-bsc/paging.c
M tests/timer.vty
3 files changed, 19 insertions(+), 11 deletions(-)