lei: retry_reopen on read-only Xapian access

lei: retry_reopen on read-only Xapian access

Xapian DBs may be modified by a parallel process while we're
reading it, and Xapian's MVCC model places the burden on readers
to retry operations.

We'll also have retry_reopen croak instead of die on errors,
which ought to help us track down some "Document not found"
errors I've occasionally seen when using "lei <q|up>".