Ref: Add constant for undefined update index

Ref: Add constant for undefined update index

Code that creates references and wants to support ref dbs with and
without update indexes needs to set this value. This leds to a
proliferation of "-1" in the code base.

Make the "undefined" value a constant in the ref interface.

Change-Id: I2622a37536a84b4a4036dd55792e185486fa0628
Signed-off-by: Ivan Frade <ifrade@google.com>