private static <T> int countEntries(Map<String, List<T>> value) {
        int entries = 0;
        for (Map.Entry<String, List<T>> property : value.entrySet())
            entries += property.getValue().size();
        return entries;
    }
Encodes a multi-map as binary.
Does nothing if the value is null.

Returns the number of maps encoded - 0 or 1
container-search/src/main/java/com/yahoo/fs4/MapEncoder.java