fix(services/alert): Improve JSON serialization performance

Uses mailru/easyjson to reduce the CPU and memory cost of serializing
the `TopicState` and `EventState` structs

Fixes #1971

Gopkg.toml

@@ -1,4 +1,8 @@
-required = ["github.com/benbjohnson/tmpl","github.com/golang/protobuf/protoc-gen-go"]
+required = [
+    "github.com/benbjohnson/tmpl",
+    "github.com/golang/protobuf/protoc-gen-go",
+    "github.com/mailru/easyjson/easyjson",
+]
 
 [prune]
   unused-packages = true
@@ -9,6 +13,10 @@ required = ["github.com/benbjohnson/tmpl","github.com/golang/protobuf/protoc-gen
   branch = "master"
   name = "github.com/davecgh/go-spew"
 
+[[constraint]]
+  branch = "master"
+  name = "github.com/mailru/easyjson"
+
 [[constraint]]
   branch = "master"
   name = "github.com/evanphx/json-patch"
@@ -57,3 +65,11 @@ required = ["github.com/benbjohnson/tmpl","github.com/golang/protobuf/protoc-gen
   name = "github.com/Azure/go-autorest"
   revision = "a2fdd780c9a50455cecd249b00bdc3eb73a78e31"
 
+[[override]]
+  name= "gopkg.in/fsnotify.v1"
+  revision = "629574ca2a5df945712d3079857300b5e4da0236"
+  source = "git@github.com:fsnotify/fsnotify"
+
+[[override]]
+  name= "github.com/mailru/easyjson"
+  revision = "3fdea8d05856a0c8df22ed4bc71b3219245e4485"

services/alert/dao.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/influxdata/kapacitor/alert"
 	"github.com/influxdata/kapacitor/services/storage"
+	"github.com/mailru/easyjson/jlexer"
 	"github.com/pkg/errors"
 )
 
@@ -243,11 +244,13 @@ type TopicStateDAO interface {
 
 const topicStateVersion = 1
 
+//easyjson:json
 type TopicState struct {
 	Topic       string                `json:"topic"`
 	EventStates map[string]EventState `json:"event-states"`
 }
 
+//easyjson:json
 type EventState struct {
 	Message  string        `json:"message"`
 	Details  string        `json:"details"`
@@ -265,8 +268,9 @@ func (t TopicState) MarshalBinary() ([]byte, error) {
 }
 
 func (t *TopicState) UnmarshalBinary(data []byte) error {
-	return storage.VersionJSONDecode(data, func(version int, dec *json.Decoder) error {
-		return dec.Decode(&t)
+	return storage.VersionEasyJSONDecode(data, func(version int, dec *jlexer.Lexer) error {
+		t.UnmarshalEasyJSON(dec)
+		return dec.Error()
 	})
 }