MapDefinition getMapDefinition(final UID uid) {
        // If we know the uid then the mapping should already be in the map
        final MapDefinition mapDefinition = uidToMapDefinitionMap.get(uid);
        return Objects.requireNonNull(mapDefinition, () -> "We should have a mapDefinition for UID " + uid);
    }
@return The map definition UIDs loaded so far
stroom-pipeline/src/main/java/stroom/pipeline/refdata/store/offheapstore/OffHeapStagingStore.java