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