Devmapper: parallelize device close/deactivate

waitClose will wait for one second and then timeout when closing a
device.  If for whatever reason 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.