dnsproxy: Start tracking the end time of the TTL

dnsproxy: Start tracking the end time of the TTL

In order to be able to report an accurate TTL to our client (see next patch
to actually do that), we are going to need to track the end point in time
for this TTL. This also allows us to track separately how long an entry
is valid versus how long we want to cache the entry.

This patch also adds some logic to group end times of the various
cache entries to logical boundaries (10 and 30 seconds); this will
cause cache entries to expire together, which is more efficient in
power/performance, and also later when we add refreshing logic.