azureblob: raise --azureblob-upload-concurrency to 16 by default

After speed testing it was discovered that upload speed goes up pretty
much linearly with upload concurrency. This patch changes the default
from 4 to 16 which means that rclone will use 16 * 4M = 64M per
transfer which is OK even for low memory devices.

This adds a note that performance may be increased by increasing
upload concurrency.

See: https://forum.rclone.org/t/performance-of-rclone-vs-azcopy/27437/9

backend/azureblob/azureblob.go

@@ -157,7 +157,7 @@ raise this to 64. Note that this will use more memory.
 Note that chunks are stored in memory and there may be up to
 "--transfers" * "--azureblob-upload-concurrency" chunks stored at once
 in memory.`,
-			Default:  4,
+			Default:  16,
 			Advanced: true,
 		}, {
 			Name: "list_chunk",