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/erratic/README.md

@@ -37,6 +37,10 @@ In case of a zone transfer and truncate the final SOA record *isn't* added to th
 
 This plugin implements dynamic health checking. For every dropped query it turns unhealthy.
 
+## Ready
+
+This plugin reports readiness to the ready plugin.
+
 ## Examples
 
 ~~~ corefile

plugin/erratic/ready.go

@@ -0,0 +1,13 @@
+package erratic
+
+import "sync/atomic"
+
+// Ready returns true if the number of received queries is in the range [3, 5). All other values return false.
+// To aid in testing we want to this flip between ready and not ready.
+func (e *Erratic) Ready() bool {
+	q := atomic.LoadUint64(&e.q)
+	if q >= 3 && q < 5 {
+		return true
+	}
+	return false
+}