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/kubernetes/ns_test.go

@@ -3,6 +3,8 @@ package kubernetes
 import (
 	"testing"
 
+	"github.com/coredns/coredns/plugin/pkg/watch"
+
 	api "k8s.io/api/core/v1"
 	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
@@ -18,6 +20,9 @@ func (APIConnTest) SvcIndexReverse(string) []*api.Service { return nil }
 func (APIConnTest) EpIndex(string) []*api.Endpoints       { return nil }
 func (APIConnTest) EndpointsList() []*api.Endpoints       { return nil }
 func (APIConnTest) Modified() int64                       { return 0 }
+func (APIConnTest) SetWatchChan(watch.Chan)               {}
+func (APIConnTest) Watch(string) error                    { return nil }
+func (APIConnTest) StopWatching(string)                   {}
 
 func (APIConnTest) ServiceList() []*api.Service {
 	svcs := []*api.Service{