Lucene: close sub-indexes in parallel

The open and closed sub-indexes may each take a while to close as they
flush and commit writes. They are completely independent, so close
them in parallel, reusing the existing indexing executor, which
shouldn't be doing much else during shutdown.

Change-Id: I3c56a972d19c793e2b8601727c03b6e7bf53e77d
3 files changed