Support placeholder values in BsonReader and BsonWriter

[jyemin]

This is an attempt to add support for placeholder values in JSON, so that our Hibernate dialect can support native queries like

{ 
   aggregate: "book",
   pipeline: [
       {
          $match: {
              title: ?
          }
       }
   ]
}

instead of the less intuitive:

{ 
   aggregate: "book",
   pipeline: [
       {
          $match: {
              title: {$undefined: true}
           }
       }
   ]
}

It could also be used for non-native queries, but it's less important there. It would be more to make the query logs look more sensible, i.e.

DEBUG o.h.p.entity.AbstractEntityPersister -  Insert (0): {"insert": "books", "documents": [{"author": ?, "publishYear": ?, "title": ?, "_id": ?}]}
DEBUG o.h.p.entity.AbstractEntityPersister -  Update (0): {"update": "books", "updates": [{"q": {"_id": {"$eq": ?}}, "u": {"$set": {"author": ?, "publishYear": ?, "title": ?}}, "multi": true}]}
DEBUG o.h.p.entity.AbstractEntityPersister -  Delete (0): {"delete": "books", "deletes": [{"q": {"_id": {"$eq": ?}}, "limit": {"$numberInt": "0"}}]}

instead of

DEBUG o.h.p.entity.AbstractEntityPersister -  Insert (0): {"insert": "books", "documents": [{"authorFirstName": {"$undefined": true}, "authorLastName": {"$undefined": true}, "publishYear": {"$undefined": true}, "title": {"$undefined": true}, "_id": {"$undefined": true}}]}
DEBUG o.h.p.entity.AbstractEntityPersister -  Update (0): {"update": "books", "updates": [{"q": {"_id": {"$eq": {"$undefined": true}}}, "u": {"$set": {"authorFirstName": {"$undefined": true}, "authorLastName": {"$undefined": true}, "publishYear": {"$undefined": true}, "title": {"$undefined": true}}}, "multi": true}]}
DEBUG o.h.p.entity.AbstractEntityPersister -  Delete (0): {"delete": "books", "deletes": [{"q": {"_id": {"$eq": {"$undefined": true}}}, "limit": {"$numberInt": "0"}}]}

The problem is that in order to fit in the BsonReader/BsonWriter framework, the change is quite invasive, including

• Adding writePlaceholder methods to BsonWriter
• Adding readPlaceholder methods to BsonReader
• Adding a BsonPlaceholder subclass to BsonValue
• Adding a PLACEHOLDER value to the BsonType enum
• Adding a BsonPlaceholderCodec
• Added asPlaceholder and isPlaceholder to BsonValue
• Adding doWritePlaceholder method to AbstractBsonWriter and subclasses, including one in BsonBinaryWriter that necessarily throws UnsupportedOperationException since placeholder is not a real BSON type
• Adding doReadPlaceholder method to AbstractBsonReader and subclasses, including one in BsonBinaryReader that necessarily throws UnsupportedOperationException since placeholder is not a real BSON type

It would be hard to justify all these changes, but I thought it would be interesting for people to look at.

[jyemin]

This part was easy, since we already support raw strings dues to the shell syntax