Feature: disable COW

[kolayne]

The overlay filesystem imposes some limitations, for example, it has undefined behavior if the files in lowerdirs change while it is mounted.

The suggested feature is to give users an option to disable copy-on-write so that a straightforward recursive copy of the base directory is performed instead

[kolayne]

Possible implementation that would be consistent with the mountpoint-dir-is-a-mount-point invariant (which the plugin needs do detect non-cleanly unmounted volumes, e.g., in case of reboot):

• for a volatile no-COW volume, mount tmpfs onto mountpoint/
• for a non-volatile no-COW volume, do a bind mount of mountpoint/ onto itself