Fix GPMONGODB-257.

New AssociationIndexer.preIndex method added, which allows embedding indices directly in the document
before it is persisted, if appropriate to the store type (useful for document databases like Mongo
or CouchDB).

Mongo tests passed, indicating no regressions. New test cases weren't possible, since there is
no way to intercept the mongo driver to ensure that only the expected persistence operations are performed.