server: try to consistently use (Name.t, string) to represent watches
server: try to consistently use (Name.t, string) to represent watches

This applies to both watch events and watch registrations.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

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