* Migrate FSTNullValue to FSTDelegateValue
* Migrate FSTIntegerValue to FSTDelegateValue
* Migrate FSTDoubleValue to FSTDelegateValue
* Migrate FSTGeoPointValue to FSTDelegateValue
* Remove stale references to DatabaseId
* Have Firestore take the DatabaseId directly
* Allow tests to default the database implicitly
* Have FSTSerializerBeta take DatabaseId by value
* Have FSTUserDataConverter take DatabaseId by value
* Make FIRFirestore.databaseID a const reference
* Make FSTFirestoreClient.databaseID a const reference
* Have FSTReferenceValue take DatabaseId by value
* Have DatabaseInfo take DatabaseId by value
* Have Serializer take DatabaseId by value
* Fix C++ ReferenceValue
* Take DatabaseId by value
* Reverse order to match Android
* Rename `reference` to `key`
* Split data conversion tests into FSTUserDataConverterTests
* Add TestUtil Value/Object to match Java
* Port Java FieldValue tests
* FieldValue::Map -> ObjectValue (#3047)