New feature: CACHE

[amaklp]

For writes I think the mount command with the new vfs-cache modes will improve much the writes.

Here’s a useful comment from ncw

Cache->Crypt Remote vs Crypt Remote with "--cache-mode" VFS

There are two caches in rclone at the moment, both under development! I can see them merging in the future though. First there is the cache backend which can be used with rclone mount, or with any rclone command. Second there is the VFS cache. The VFS is only used for rclone mount or rclone serve webdav or rclone serve http. The VFS interprets the --cache-mode flag - I think I should probably rename that to --vfs-cache-mode to save confusion with the backend cache. Due to the way object st…