Remove BaseDataProxy.*by_mongo_name methods

[lafrech]

Those are private methods only ever used internally for the _id. We could replace them with a more specific method.

[lafrech]

Here's a plan:

• Add _id getter/setter to BaseDataProxy (no specific logic, just avoid accessing private _data)
• Use this instead of set/get_by_mongo_name from Document class
• Remove *_by_mongo_name from BaseDataProxy

No need to catch FieldNotLoadedError because

• the "ID field not loaded" case is not really supported (committing would fail with an UpdateError if committing a doc with ID loaded)
• we're removing partial anyway (Remove partial load feature #256)

[lafrech]

Alternatively, the document could be be responsible for the id field <-> _id relation:

• No getter/setter in BaseDataProxy
• Add pk_field property to Document

Use this property to

• prevent modifying ID field (Raise AlreadyCreatedError when modifying pk on a created document #173)
• set ID from drivers and expose pk value (currently using set/get_by_mongo_name)