Client and server now map the same buffers

  - make a Location.t which represents the location of a ring
  - Location.of_lengths picks two suitable non-overlapping locations
  - the server decides the locations and the client uses the
    server-provided settings

Signed-off-by: David Scott <dave.scott@citrix.com>
Regenerate oasis

Signed-off-by: David Scott <dave.scott@citrix.com>