* provider/scaleway: fix scaleway_volume_attachment with count > 1
since scaleway requires servers to be powered off to attach volumes to, we need
to make sure that we don't power down a server twice, or power up a server while
it's supposed to be modified.
sadly terraform doesn't seem to sport serialization primitives for usecases like
this, but putting the code in question behind a `sync.Mutex` does the trick, too
fixes#9417
* provider/scaleway: use mutexkv to lock per-resource
following @dcharbonnier suggestion. thanks!
* provider/scaleway: cleanup waitForServerState signature
* provider/scaleway: store serverID in var
* provider/scaleway: correct imports
* provider/scaleway: increase timeouts
d9a2e0dbb3