Add support to partial re-index in the MassIndexer

Currently I can re-index by put the entry again. But it will make it replicated again in distributed cache with backups. It is necessary to provide a method to re-index the entry.
Maybe like this:
MassIndexer.reindex(Object key...)

It would be useful for relational model to re-index the parent while change the child when the parent refers to the child by key. In this situation, the parent is not modified at all. So a re-index is needed.