SerializerPropertyMetadataFactory overwrites readableLink and writableLink metadata

[oxan]

The SerializerPropertyMetadataFactory overwrites the readableLink and writableLink property metadata with its own computed value. This is unfortunate, as it makes setting them to false using an annotation or XML/YAML configuration impossible. Manually setting these properties to false is useful in situations like #1001 (entity using serialization groups references itself, and wants to serialize self-reference as an IRI instead of embedded document).

Also related to #1980.

[soyuka]

👍 let's keep discussing this in #1980 but user-defined metadata should totally override the rest.