Periodically reinitialize RRDP sessions to snapshot at random intervals

Periodically reinitialize RRDP sessions to snapshot at random intervals

It is technically possible for a series of RRDP deltas and a snapshot
to diverge. An RRDP server could distribute files via Deltas and then
forget about those files, causing copies to remain stuck in the caches
of RRDP clients. Resetting RRDP sessions once every few weeks helps with
garbage collection.

In week 0 the probability of triggering re-initialization is ~0.025% and
doubles every week, in week 11 its 50% and always after week 12. Thus,
RPs will reinitialize at least once every 3 months.

OK tb@ claudio@