plugin/cancel: add context cancelation plugin (#2711)

* plugin/cancel: add context cancelation plugin

Per review comments on #2704, move this into a plugin that gets called.
Add the most minimal plugin, tests and documenation.

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

* plugin/cache: add timeout option

review feedback: add option to set custom timeout.

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

* spelling

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

plugin/cancel/README.md

@@ -0,0 +1,45 @@
+# cancel
+
+## Name
+
+*cancel* - a plugin that cancels a request's context after 5001 milliseconds.
+
+## Description
+
+The *cancel* plugin creates a canceling context for each request. It adds a timeout that gets
+triggered after 5001 milliseconds.
+
+The 5001 number is chosen because the default timeout for DNS clients is 5 seconds, after that they
+give up.
+
+A plugin interested in the cancellation status should call `plugin.Done()` on the context. If the
+context was canceled due to a timeout the plugin should not write anything back to the client and
+return a value indicating CoreDNS should not either; a zero return value should suffice for that.
+
+~~~ txt
+cancel [TIMEOUT]
+~~~
+
+* **TIMEOUT** allows setting a custom timeout. The default timeout is 5001 milliseconds (`5001 ms`)
+
+## Examples
+
+~~~ corefile
+. {
+    cancel
+    whoami
+}
+~~~
+
+Or with a custom timeout:
+
+~~~ corefile
+. {
+    cancel 1s
+    whoami
+}
+~~~
+
+## Also See
+
+The Go documentation for the context package.