gnus: use mbsync instead of offlineimap

offlineimap has more features but it is quite slower and not energy
efficient on a laptop (active polling every 50ms). We lose the great
offlineimap-mode but we replace it with a more manual mode for mbsync.
gnus: don't use mbsync.el
gnus: provide a very thin wrapper around mbsync
gnus: add mbsync status in status line
gnus: ensure only one instance of mbsync is running
gnus: fix how prefix argument is handled by mbsync wrapper