Upstream: copy upstream zone DNS valid time during config reload.

Upstream: copy upstream zone DNS valid time during config reload.

Previously, all upstream DNS entries would be immediately re-resolved
on config reload.  With a large number of upstreams, this creates
a spike of DNS resolution requests.  These spikes can overwhelm the
DNS server or cause drops on the network.

This patch retains the TTL of previous resolutions across reloads
by copying each upstream's name's expiry time across configuration
cycles.  As a result, no additional resolutions are needed.