plugin/ready: fix starts and restarts (#2814)

Add OnRestartFailed to the ready plugin and some various cleanups.

Document slightly better how things are supposed to work with multiple
`ready`'s in the multiple Server Blocks.

All manually tested with this Corefile:
~~~
. {
    log
    ready
}

example.org {
    log
    chaos
    ready
}
~~~
And then `kill -SIGUSR1` and curling the ready endpoint. This works
well, the FailedReload is triggered by adding a syntax error in the
Corefile.

See #2659

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/ready/ready.go

@@ -30,10 +30,6 @@ type ready struct {
 }
 
 func (rd *ready) onStartup() error {
-	if rd.Addr == "" {
-		rd.Addr = defAddr
-	}
-
 	ln, err := net.Listen("tcp", rd.Addr)
 	if err != nil {
 		return err
@@ -62,8 +58,6 @@ func (rd *ready) onStartup() error {
 	return nil
 }
 
-func (rd *ready) onRestart() error { return rd.onFinalShutdown() }
-
 func (rd *ready) onFinalShutdown() error {
 	rd.Lock()
 	defer rd.Unlock()
@@ -77,5 +71,3 @@ func (rd *ready) onFinalShutdown() error {
 	rd.done = false
 	return nil
 }
-
-const defAddr = ":8181"