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",

docs/content/azureblob.md

@@ -81,6 +81,14 @@ key.  It is stored using RFC3339 Format time with nanosecond
 precision.  The metadata is supplied during directory listings so
 there is no overhead to using it.
 
+### Performance
+
+When uploading large files, increasing the value of
+`--azureblob-upload-concurrency` will increase performance at the cost
+of using more memory. The default of 16 is set quite conservatively to
+use less memory. It maybe be necessary raise it to 64 or higher to
+fully utilize a 1 GBit/s link with a single file transfer.
+
 ### Restricted filename characters
 
 In addition to the [default restricted characters set](/overview/#restricted-characters)