S57: make the driver thread-safe (move part of the state and methods of S57ClassRegistrar to a new class S57ClassContentExplorer that is instanciated by each OGRS57DataSource

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26333 f0d54148-0727-0410-94bb-9a71ac55c965