* pkg/up: implement backoff

Every 2nd failure we double the interval until we hit 4 * interval. This
to have some sort of backoff, esp when a large cluster of coredns shares
an upstream (original intent of up package) they will hammer the
upstream. This put some back pressure on that.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/pkg/up/up.go
Co-Authored-By: miekg <miek@miek.nl>