A solicitation loop could result for two cases:

1) a router sent DNS information, then removed that information without
sending it with lifetime=0
2) two routers exist, one sending DNS information and the other not, and
the first router which sends DNS information disappears

In these cases a solicitation would be generated when the DNS information
reached 1/2 its lifetime.  A router would then reply to the solicitation
without DNS information, which would then trigger another lifetime check,
which finds that the DNS info is still 1/2 lifetime.  Which triggers
another solicitation, etc.

Fix this by ensuring that a solicitation is never sent less than
rtr_solicitation_interval seconds after the last one.