add function to lock a domain

add function to lock a domain

This can be used to execute code on an 'action domain' basis.
E.g.: if there are actions that cannot be run simultaneously even if
they, for example, don't access a common file and maybe also spread
across different packages we can now secure the consistence of said
actions on an 'action domain' basis.

The need to use a dirty hack like cfs_lock_storage with some
arbitrary storage name becomes obsolete. Also the code behaviour
and meaning becomes clearer.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>