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

plugin/forward/setup.go

@@ -62,25 +62,14 @@ func setup(c *caddy.Controller) error {
 
 // OnStartup starts a goroutines for all proxies.
 func (f *Forward) OnStartup() (err error) {
-	if f.hcInterval == 0 {
-		for _, p := range f.proxies {
-			p.host.fails = 0
-		}
-		return nil
-	}
-
 	for _, p := range f.proxies {
-		go p.healthCheck()
+		p.start(f.hcInterval)
 	}
 	return nil
 }
 
 // OnShutdown stops all configured proxies.
 func (f *Forward) OnShutdown() error {
-	if f.hcInterval == 0 {
-		return nil
-	}
-
 	for _, p := range f.proxies {
 		p.close()
 	}
@@ -88,9 +77,7 @@ func (f *Forward) OnShutdown() error {
 }
 
 // Close is a synonym for OnShutdown().
-func (f *Forward) Close() {
-	f.OnShutdown()
-}
+func (f *Forward) Close() { f.OnShutdown() }
 
 func parseForward(c *caddy.Controller) (*Forward, error) {
 	f := New()
@@ -140,8 +127,8 @@ func parseForward(c *caddy.Controller) (*Forward, error) {
 			}
 
 			// We can't set tlsConfig here, because we haven't parsed it yet.
-			// We set it below at the end of parseBlock.
-			p := NewProxy(h)
+			// We set it below at the end of parseBlock, use nil now.
+			p := NewProxy(h, nil /* no TLS */)
 			f.proxies = append(f.proxies, p)
 		}
 
@@ -200,17 +187,11 @@ func parseBlock(c *caddy.Controller, f *Forward) error {
 			return fmt.Errorf("health_check can't be negative: %d", dur)
 		}
 		f.hcInterval = dur
-		for i := range f.proxies {
-			f.proxies[i].hcInterval = dur
-		}
 	case "force_tcp":
 		if c.NextArg() {
 			return c.ArgErr()
 		}
 		f.forceTCP = true
-		for i := range f.proxies {
-			f.proxies[i].forceTCP = true
-		}
 	case "tls":
 		args := c.RemainingArgs()
 		if len(args) != 3 {