Call the extra_watch_callback on a different thread than the dispatcher
Call the extra_watch_callback on a different thread than the dispatcher

Otherwise, a call to e.g. Xs.read will deadlock if you try to do it
from the callback.

Signed-off-by: Jon Ludlam <e7e3380887a8f95cc9dc4f0d51dedc7e849a287a@eu.citrix.com>

mirage/ocaml-xenstore-server,mirage/ocaml-xenstore-server,djs55/ocaml-xenstore-server