Remove the remaining fields from *app.Server (#19113)

We move these fields to Channels:
```
uploadLockMapMut sync.Mutex
uploadLockMap    map[string]bool
imgDecoder *imaging.Decoder
imgEncoder *imaging.Encoder
dndTaskMut sync.Mutex
dndTask    *model.ScheduledTask
```

I think this PR should conclue the initial phase
of migrating stuff from Server to Channels.

The remaining task would be to focus on continue
to create the remaining services from the
common things like users, teams, push notifications,
clustering for other products to consume.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/87df1e15-588e-49ff-8bd1-ffa9651b8c82

```release-note
NONE
```

app/channels.go

@@ -4,9 +4,11 @@
 package app
 
 import (
+	"runtime"
 	"sync"
 	"sync/atomic"
 
+	"github.com/mattermost/mattermost-server/v6/app/imaging"
 	"github.com/mattermost/mattermost-server/v6/app/request"
 	"github.com/mattermost/mattermost-server/v6/einterfaces"
 	"github.com/mattermost/mattermost-server/v6/model"
@@ -45,6 +47,18 @@ type Channels struct {
 	Compliance       einterfaces.ComplianceInterface
 	DataRetention    einterfaces.DataRetentionInterface
 	MessageExport    einterfaces.MessageExportInterface
+
+	// These are used to prevent concurrent upload requests
+	// for a given upload session which could cause inconsistencies
+	// and data corruption.
+	uploadLockMapMut sync.Mutex
+	uploadLockMap    map[string]bool
+
+	imgDecoder *imaging.Decoder
+	imgEncoder *imaging.Encoder
+
+	dndTaskMut sync.Mutex
+	dndTask    *model.ScheduledTask
 }
 
 func init() {
@@ -55,8 +69,9 @@ func init() {
 
 func NewChannels(s *Server) (*Channels, error) {
 	ch := &Channels{
-		srv:        s,
-		imageProxy: imageproxy.MakeImageProxy(s, s.httpService, s.Log),
+		srv:           s,
+		imageProxy:    imageproxy.MakeImageProxy(s, s.httpService, s.Log),
+		uploadLockMap: map[string]bool{},
 	}
 	// We are passing a partially filled Channels struct so that the enterprise
 	// methods can have access to app methods.
@@ -75,6 +90,20 @@ func NewChannels(s *Server) (*Channels, error) {
 		ch.AccountMigration = accountMigrationInterface(New(ServerConnector(ch)))
 	}
 
+	var imgErr error
+	ch.imgDecoder, imgErr = imaging.NewDecoder(imaging.DecoderOptions{
+		ConcurrencyLevel: runtime.NumCPU(),
+	})
+	if imgErr != nil {
+		return nil, errors.Wrap(imgErr, "failed to create image decoder")
+	}
+	ch.imgEncoder, imgErr = imaging.NewEncoder(imaging.EncoderOptions{
+		ConcurrencyLevel: runtime.NumCPU(),
+	})
+	if imgErr != nil {
+		return nil, errors.Wrap(imgErr, "failed to create image encoder")
+	}
+
 	// Setup routes.
 	pluginsRoute := ch.srv.Router.PathPrefix("/plugins/{plugin_id:[A-Za-z0-9\\_\\-\\.]+}").Subrouter()
 	pluginsRoute.HandleFunc("", ch.ServePluginRequest)
@@ -109,6 +138,13 @@ func (ch *Channels) Start() error {
 
 func (ch *Channels) Stop() error {
 	ch.ShutDownPlugins()
+
+	ch.dndTaskMut.Lock()
+	if ch.dndTask != nil {
+		ch.dndTask.Cancel()
+	}
+	ch.dndTaskMut.Unlock()
+
 	return nil
 }