CA-101580: rrdd: watch xenstore instead of polling

xcp-rrdd is interested in domains' data/meminfo_free values.
In one example, polling xenstore for these values took about
30 seconds for 700 VMs. Instead of polling, use watches.

Imported from xen-api.git dc787f75ee153c3d0690540b155c9e2badf78163

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Imported-by: John Else <john.else@citrix.com>