Allow efficient search by blob key

We need to be able to efficiently search all documents containing a given blob key:

• to check if a blob is not referenced anymore and can be deleted, after document deletion for instance (this is a limited form of blob GC),
• to rename a temporary blob key after we've computed a new key based on the content digest.

To do this, we will add to DBS documents a new internal denormalized list property, ecm:blobKeys, containing the list of all blob keys, and provide a way to query it efficiently using NXQL.

As a side benefit this will improve the efficiency of the blob GC as it will be sufficient to request this single property.