COR-329 : Event User Model approach for MSExcelDocumentReader and MSXExcelDocumentReader

Fix description:
* Use Apache POI's Event User Model to have lower memory footprint with big Excel files
* Limit the parsing to 5000 cells. Only index text cells of more than 2 characters.
* Improve POIPropertiesReader to have a lower memory footprint in getProperties with XLSX documents
COR-329: Streaming parser for MSXPPTDocumentReader

Fix description:
* Implement streaming model to get properties and content of Microsoft Powerpoint files (OOXML).
* Index the content of the first 500 slides.