Don't throw an exception when trying to re-add an index version that
previously existed. Implementations may want to swap out an index with
another of the same version as a result of some state change. As with
other pieces of the IndexCollection abstraction, callers are already
responsible for initializing/tearing down their implementations, so
return any replaced index.
Change-Id: I1675e135fec7d8891c112ddc18a8e75eeca14f5b
35de7d3944