In I1b3c5ba0 ChangeIndex.replace was modified to return a future, but
ChangeIndexer.Task was not changed to call those futures before
returning.
In retrospect this was a mistake anyway. The only path by which these
methods are currently called is from ChangeIndexerImpl, which turns
around and wraps them in a Callable and submits them to the same
executor, tying up twice as many threads as necessary.
Similarly, to avoid additional fanout in ChangeIndexerImpl, we should
not write to multiple index versions in parallel, so remove the TODO
to try that. The current behavior slows down individual writes for the
relatively short period of an online version upgrade, but that is
acceptable given the improved thread utilization overall.
Change-Id: I5fff470214ecd69a261dec1c10b9f7bdd0a1907e