Improve performance of document fetching from ES

Right now it is less efficient to fetch documents from ES than from VCS.

The reason is that org.nuxeo.elasticsearch.io.JSONDocumentReader deserialize the json payload sent by ES into instances of DocumentModelImpl using the setPropertyValue method. Doing that means for each document and each property, there is a call to the VCS to fetch the data model. In the end, the data are actually fetched twice, one from ES, one from the VCS.

Possible solutions to explore in order to improve perfs include (but are not limited to):

• replacing DocumentModelImpl by SimpleDocument
• Put the metadata directly in a Map and avoid calling setPropertyValue