paging: Split paging queue into 2 queues: initial and retrans

Initial requests: paging requests which haven't been yet transmitted
Retrans requests: paging requests which have already been transmitted at
least once.
Until now one queue was used (llist) to store both. The initial requests
were stored at the start of the queue in FIFO order. After the last
initial requests, the retrans requests followed also in FIFO.

This ordering was used in order to prioritze scheduling of initial
paging requests over retransmit paging requests.
In the end, having both types in the same list only make code handling
the list more complex.
Hence, this patch splits the shared llist into 2 llists.

Related: SYS#6200
Change-Id: Ib68f2169e3790aea4ac77ec20ad79f242b7c2747
---
M include/osmocom/bsc/paging.h
M src/osmo-bsc/bsc_vty.c
M src/osmo-bsc/paging.c
M tests/paging/paging_test.c
M tests/paging/paging_test.ok
5 files changed, 504 insertions(+), 241 deletions(-)