s3: change --s3-upload-concurrency default to 4 to increase perfomance #2772

Increasing the --s3-upload-concurrency to 4 (from 2) gives an
additional 45% throughput at the cost of 10MB extra memory per transfer.

After testing the upload perfoc

backend/s3/s3.go

@@ -585,7 +585,7 @@ concurrently.
 If you are uploading small numbers of large file over high speed link
 and these uploads do not fully utilize your bandwidth, then increasing
 this may help to speed up the transfers.`,
-			Default:  2,
+			Default:  4,
 			Advanced: true,
 		}, {
 			Name: "force_path_style",

docs/content/s3.md

@@ -919,7 +919,7 @@ this may help to speed up the transfers.
 - Config:      upload_concurrency
 - Env Var:     RCLONE_S3_UPLOAD_CONCURRENCY
 - Type:        int
-- Default:     2
+- Default:     4
 
 #### --s3-force-path-style