[Lucene] Payload一般只用于过滤、打分、排序等
我原以为可以search阶段直接把特定的payload取出来，然后打印一下，但google了很久，似乎没有直接的API.
Payload可能本来就不适用于这种用况。 Lucene in Action说，
引用"... use it during search, either to decide which documents are included in the search results or to alter how matched documents are scored or sorted"
