GEOS-5693: Make app-schema joining the default implementation.

- fix broken build from last commit (non database backends should ignore default joining setting).

Really fix last commit.
Conflicts:
	modules/extension/app-schema/app-schema/src/main/java/org/geotools/data/complex/config/AppSchemaDataAccessConfigurator.java
[GEOT-4407] temp_alias_used_for_filter causes error with H2 database
GEOT-4417: App-schema joining with toXlinkHref results in some missing
nodes.
GEOT-4430: App schema joining with duplicate ORDER BY causes SQL Server exception
GEOT-4460: Using functions in linkElement causes WMS error