xenstore: add per-node generation counter

xenstore: add per-node generation counter

In order to be able to support reading the list of a node's children in
multiple chunks (needed for list sizes > 4096 bytes) without having to
allocate a temporary buffer we need some kind of generation counter for
each node. This will help to recognize a node has changed between
reading two chunks.

As removing a node and reintroducing it must result in different
generation counts each generation value has to be globally unique. This
can be ensured only by using a global 64 bit counter.

For handling of transactions there is already such a counter available,
it just has to be expanded to 64 bits and must be stored in each
modified node.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>