lei reindex: avoid forking worker process

lei reindex: avoid forking worker process

Instead of spawning a potentially expensive dedicated process to
feed numeric docids to the lei/store process, we can instead
rely on requeue + event_step and the DS event loop of the
lei/store working to iterate through the query list.

In addition to saving memory from forking, handling iteration
entirely within one process saves some syscalls and IPC
overhead.

We'll also flesh out lei-reindex.t a bit to test reindex
explicitly, even if lei-index.t already tests it.