Always close IndexReaders in LuceneIndex.mutate

For #761 (0.4.x edition)

This is a reimplementation of 0.5.1-SNAPSHOT's
5f562761e6d8d9dcb35d8c0eeacef91f0bfe74da on 0.4.5-SNAPSHOT.  The
affected bits were different enough between the two versions that it
was easier to rewrite than resolve the conflicted cherry pick.