Support deleting Odinson docs from index

[myedibleenso]

Currently, there is not an easy way to delete an OdinsonDocument from an index. We should add support for deleting a block of documents (sentences).

Proposal

1. OdinsonIndex.indexOdinsonDoc and its implementations should return the Lucene doc ID of the block's parent.

• See also IncrementalOdinsonIndex.indexOdinsonDoc

2. OdinsonIndexReader should have a helper method (ex. deleteBlockByParentId) to delete a block of documents (sentences) from the index based on the Lucene doc ID of a block's parent (relates to OdinsonDocumentIndexReader.reader.deleteDocument()).

Issues

• Deleting an OdinsonDocument should mean removing all Lucene documents created from that OdinsonDocument.
• OdinsonIndexWriter.mkNestedDocument() creates a Lucene document, but that document does not include a field with OdinsonIndexWriter.DOC_ID_FIELD. This makes it easy to overlook when sweeping for all Lucene docs created from an OdinsonDocument.

[myedibleenso]

Relates to #279 (NOTE: we likely won't support all of those endpoints in the REST API).