Watch feature (#1527)

* Add part 1 watch functionality. (squashed)

* add funcs for service/endpoint fqdns

* add endpoints watch

* document exposed funcs

* only send subset deltas

* locking for watch map

* tests and docs

* add pod watch

* remove debugs prints

* feedback part 1

* add error reporting to proto

* inform clients of server stop+errors

* add grpc options param

* use proper context

* Review feedback:
 * Removed client (will move to another repo)
 * Use new log functions
 * Change watchChan to be for string not []string
 * Rework how k8s plugin stores watch tracking info to simplify
 * Normalize the qname on watch request

* Add blank line back

* Revert another spurious change

* Fix tests

* Add stop channel.
Fix tests.
Better docs for plugin interface.

* fmt.Printf -> log.Warningf

* Move from dnsserver to plugin/pkg/watch

* gofmt

* remove dead client watches

* sate linter

* linter omg

plugin/pkg/watch/watch.go

@@ -0,0 +1,23 @@
+package watch
+
+// Chan is used to inform the server of a change. Whenever
+// a watched FQDN has a change in data, that FQDN should be
+// sent down this channel.
+type Chan chan string
+
+// Watchable is the interface watchable plugins should implement
+type Watchable interface {
+	// Name returns the plugin name.
+	Name() string
+
+	// SetWatchChan is called when the watch channel is created.
+	SetWatchChan(Chan)
+
+	// Watch is called whenever a watch is created for a FQDN. Plugins
+	// should send the FQDN down the watch channel when its data may have
+	// changed. This is an exact match only.
+	Watch(qname string) error
+
+	// StopWatching is called whenever all watches are canceled for a FQDN.
+	StopWatching(qname string)
+}