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/pkg/uniq/uniq.go

@@ -28,7 +28,7 @@ func (u U) Unset(key string) {
 	delete(u.u, key)
 }
 
-// ForEach iterates for u executes f for each element that is 'todo' and sets it to 'done'.
+// ForEach iterates over u and executes f for each element that is 'todo' and sets it to 'done'.
 func (u U) ForEach() error {
 	for k, v := range u.u {
 		if v.state == todo {