Faster initial population of label scan store.

Improve write performance by refreshing the lucene searcher less often.
Refreshing less often is possible if writes are ordered so that we never
need to read our own writes during a single batch of writes. We consider
initial population as one very large batch.

A new facade LabelScanWriter expresses the batching of writes, replacing
LabelScanStore#updateAndCommit. It also mirrors the way we use LabelScanReader.