MDP-5 Cache metadata for max validity period.

Try to rebuild metadata as soon as max age is reached, but cache up to the maximum validity period on the underlying metadata document. Increase the cache period to 4 hours and maximum validity to 7 days to further increase resiliency. It is safe to bump cache period because metadata is rebuilt on ED service changes as part of replication process, so we are still responsive to changes even with longer cache periods.