ns: Dump namespaces in parallel

The main reason for this is -- dumping namespace has a lot of
points when the process just waits for something. At the same
time criu process wait for the ns dumper and doesn't dump
others.

The great example of waiting for something is setns syscall.
Very often it calls synchronize_rcu() which can be quite long.
Let other processes do smth useful while this.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>