azureblob: fix memory usage by upgrading the SDK and implementing a TransferManager

In the Azure SDK there was a bug which caused excessive memory use
when doing repeated transfers:

https://github.com/Azure/azure-storage-blob-go/issues/233

This patch updates the SDK to v0.13.0 which allowed us to implement a
custom TransferManager which integrates with rclone's memory pool.

This fixes the excessive memory consumption.

See: https://forum.rclone.org/t/ask-for-settings-recommendation-for-azureblob/21505/

backend/azureblob/azureblob.go

@@ -49,6 +49,7 @@ const (
 	storageDefaultBaseURL = "blob.core.windows.net"
 	defaultChunkSize      = 4 * fs.MebiByte
 	maxChunkSize          = 100 * fs.MebiByte
+	uploadConcurrency     = 4
 	defaultAccessTier     = azblob.AccessTierNone
 	maxTryTimeout         = time.Hour * 24 * 365 //max time of an azure web request response window (whether or not data is flowing)
 	// Default storage account, key and blob endpoint for emulator support,
@@ -1492,6 +1493,48 @@ func increment(xs []byte) {
 	}
 }
 
+// poolWrapper wraps a pool.Pool as an azblob.TransferManager
+type poolWrapper struct {
+	pool     *pool.Pool
+	bufToken chan struct{}
+	runToken chan struct{}
+}
+
+// newPoolWrapper creates an azblob.TransferManager that will use a
+// pool.Pool with maximum concurrency as specified.
+func (f *Fs) newPoolWrapper(concurrency int) azblob.TransferManager {
+	return &poolWrapper{
+		pool:     f.pool,
+		bufToken: make(chan struct{}, concurrency),
+		runToken: make(chan struct{}, concurrency),
+	}
+}
+
+// Get implements TransferManager.Get().
+func (pw *poolWrapper) Get() []byte {
+	pw.bufToken <- struct{}{}
+	return pw.pool.Get()
+}
+
+// Put implements TransferManager.Put().
+func (pw *poolWrapper) Put(b []byte) {
+	pw.pool.Put(b)
+	<-pw.bufToken
+}
+
+// Run implements TransferManager.Run().
+func (pw *poolWrapper) Run(f func()) {
+	pw.runToken <- struct{}{}
+	go func() {
+		f()
+		<-pw.runToken
+	}()
+}
+
+// Close implements TransferManager.Close().
+func (pw *poolWrapper) Close() {
+}
+
 // Update the object with the contents of the io.Reader, modTime and size
 //
 // The new object may have been created if an error is returned
@@ -1538,9 +1581,10 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 
 	putBlobOptions := azblob.UploadStreamToBlockBlobOptions{
 		BufferSize:      int(o.fs.opt.ChunkSize),
-		MaxBuffers:      4,
+		MaxBuffers:      uploadConcurrency,
 		Metadata:        o.meta,
 		BlobHTTPHeaders: httpHeaders,
+		TransferManager: o.fs.newPoolWrapper(uploadConcurrency),
 	}
 
 	// Don't retry, return a retry error instead