Add AbstractDOMSchemaService and FixedDOMSchemaService
AbstractDOMSchemaService allows more code reuse between implementations
by holding the common bits needed to interface with SchemaContextProviders.
Also use these to implement a FixedDOMSchemaService for use in situations
when the SchemaContext and the set of associated sources are known to
never change.
This also slightly simplifies ScanningSchemaSourceProvider.
JIRA: MDSAL-418
Change-Id: I554837f8ae93e5bbb741b4cb801c31b8ebfcaa03
Signed-off-by: Michael Vorburger <vorburger@redhat.com>