public static long getMinDirId(final Path parentDir) {
        return LOGGER.logDurationIfDebugEnabled(
                () ->
                        getDirIdFromRoot(parentDir, Mode.MIN),
                () ->
                        LogUtil.message("Get min dirId for {}", parentDir));
    }
Get the min id of any numerically named file found in the supplied dir.

@param parentDir The parent dir to look at.
@return The min id of all files found in a dir or 0 if non found.
stroom-state/stroom-planb-impl/src/main/java/stroom/planb/impl/data/DirUtil.java