Add *ready* plugin (#2616)

Add a ready plugin that allows plugin to signal when they are ready.
Once a plugin is ready it is not queried again.

This uses same mechanism as the health plugin: each plugin needs to
implement an interface.

Implement readines for the *erratic* plugin to aid in testing.

Add README.md and tests moduled after the health plugin; which will be
relegated to just providing process health. In similar vein to health
this is a process wide setting.

With this Corefile:
~~~
. {
    erratic
    whoami
    ready
}

bla {
    erratic
    whoami
}
~~~

ready will lead to:

~~~ sh
% curl localhost:8181/ready
% dig @localhost -p 1053 mx example.org
% curl localhost:8181/ready
OK%
~~~

Meanwhile CoreDNS logs:

~~~
.:1053
bla.:1053
2019-02-26T20:59:07.137Z [INFO] CoreDNS-1.3.1
2019-02-26T20:59:07.137Z [INFO] linux/amd64, go1.11.4,
CoreDNS-1.3.1
linux/amd64, go1.11.4,
2019-02-26T20:59:11.415Z [INFO] plugin/ready: Still waiting on: "erratic"
2019-02-26T20:59:13.510Z [INFO] plugin/ready: Still waiting on: "erratic"
~~~

*ready* can be used in multiple server blocks and will do the right
thing; query all those plugins from all server blocks for readiness.
This does a similar thing to the prometheus plugin.

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

plugin/ready/list.go

@@ -0,0 +1,48 @@
+package ready
+
+import (
+	"sort"
+	"strings"
+	"sync"
+)
+
+// list is structure that holds the plugins that signals readiness for this server block.
+type list struct {
+	sync.RWMutex
+	rs    []Readiness
+	names []string
+}
+
+// Append adds a new readiness to l.
+func (l *list) Append(r Readiness, name string) {
+	l.Lock()
+	defer l.Unlock()
+	l.rs = append(l.rs, r)
+	l.names = append(l.names, name)
+}
+
+// Ready return true when all plugins ready, if the returned value is false the string
+// contains a comma separated list of plugins that are not ready.
+func (l *list) Ready() (bool, string) {
+	l.RLock()
+	defer l.RUnlock()
+	ok := true
+	s := []string{}
+	for i, r := range l.rs {
+		if r == nil {
+			continue
+		}
+		if !r.Ready() {
+			ok = false
+			s = append(s, l.names[i])
+		} else {
+			// if ok, this plugin is ready and will not be queried anymore.
+			l.rs[i] = nil
+		}
+	}
+	if ok {
+		return true, ""
+	}
+	sort.Strings(s)
+	return false, strings.Join(s, ",")
+}