paging: Decouple retransmit period from regular worker interval

Before this patch, on each BTS a 500ms timer was used to schedule some
work, sending up to 20 paging requests verytime.

This means, however, for an initial paging request it may take up to
500ms delay to be scheduled to the BTS, which is huge.

While we still want to maintain this 500ms interval for retransmits, it
doesn't make sense to wait that much for other cases. It's far better
sending less requests (10 instead of 20) every half time (250ms instead
of 500ms), since it will spread the load and paging more over time,
allowing for other work to be done in the middle.

Change-Id: I7a1297452cc4734b6ee8c38fb94cf32f38d57c3d
---
M include/osmocom/bsc/paging.h
M src/osmo-bsc/paging.c
2 files changed, 33 insertions(+), 2 deletions(-)