plugin/forward using pkg/up (#1493)

* plugin/forward: on demand healtchecking

Only start doing health checks when we encouner an error (any error).
This uses the new pluing/pkg/up package to abstract away the actual
checking. This reduces the LOC quite a bit; does need more testing, unit
testing and tcpdumping a bit.

* fix tests

* Fix readme

* Use pkg/up for healthchecks

* remove unused channel

* more cleanups

* update readme

* * Again do go generate and go build; still referencing the wrong forward
  repo? Anyway fixed.
* Use pkg/up for doing the healtchecks to cut back on unwanted queries
  * Change up.Func to return an error instead of a boolean.
  * Drop the string target argument as it doesn't make sense.
* Add healthcheck test on failing to get an upstream answer.

TODO(miek): double check Forward and Lookup and how they interact with
HC, and if we correctly call close() on those

* actual test

* Tests here

* more tests

* try getting rid of host

* Get rid of the host indirection

* Finish removing hosts

* moar testing

* import fmt

* field is not used

* docs

* move some stuff

* bring back health_check

* maxfails=0 test

* git and merging, bah

* review

1 files changed, 2 insertions, 1 deletions

diff --git a/plugin/forward/forward_test.go b/plugin/forward/forward_test.go
index d467a0efa..26167c25d 100644
--- a/plugin/forward/forward_test.go
+++ b/plugin/forward/forward_test.go
@@ -6,6 +6,7 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/test"
 	"github.com/coredns/coredns/request"
+
 	"github.com/miekg/dns"
 )
 
@@ -18,7 +19,7 @@ func TestForward(t *testing.T) {
 	})
 	defer s.Close()
 
-	p := NewProxy(s.Addr)
+	p := NewProxy(s.Addr, nil /* not TLS */)
 	f := New()
 	f.SetProxy(p)
 	defer f.Close()