Inside batchCatalog: keep a change to shard's postings and apply it when the
batch ends, with every other change to that shard, before the catalog is written.
Returns false outside a batch, and the caller applies the change at once. Optional:
a store without it applies every change at once (issue #30).
Inside
batchCatalog: keep a change toshard's postings and apply it when the batch ends, with every other change to that shard, before the catalog is written. Returns false outside a batch, and the caller applies the change at once. Optional: a store without it applies every change at once (issue #30).