Devmapper: parallelize device close/deactivate

waitClose will wait for one second and then timeout when closing a
device.  If many devices will not close, each of those one second
waits happen serially and causes daemon shutdown to become annoyingly
slow.

This parallelizes the close/deactivate so that each device will still
have a one second timeout, but the aggregate wait for all devices
comes down to one second of wall time as well.