CA-102259: make squeezed use the new xenstore-watching interface

The only change to observational semantics is that the following actions are no longer performed atomically:

-				  Mutex.execute m
-					  (fun () ->
-						  IntSet.iter (Hashtbl.remove cache) gone;
-						  IntSet.iter (fun domid -> try ignore(get_per_domain (xc, xs) domid) with _ -> ()) arrived
-					  ) in

Now, we execute (Hashtbl.remove cache) on each disappeared domain, separately, and get_per_domain on each new domain, separately.
(cherry picked from commit 0ab61fa038a14a5fd311344144ce42da3621378a)
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.
(cherry picked from commit dc787f75ee153c3d0690540b155c9e2badf78163)
Merge pull request #1621 from thomassa/tampa-lcm-sctx-1637-rrdd-squeezed

[TAMPA] rrdd and squeezed: use new xenstore-watching, not polling.