Obtaining internal caches from HotRod causes extra registration of SourceMigrators

Internal caches such as ___protobuf_metadata always register a HotRodSourceMigrator when obtained via Hot Rod:

remotecacheManager.getCache(ProtobufMetadataManagerConstants.PROTOBUF_METADATA_CACHE_NAME)

This is because HotRod never adds internal caches to the knownCaches instance in order to check access to those caches. Regardless, it should not keep registering SourceMigrators everytime