[MorkDB] Add re_index_patterns()

[arturgontijo]

This PR:

1 - Moves Link->metta_expression field to Link-> custom_attributes["metta_expression"].
2 - Implements MorkDB::re_index_patterns() to restore S-Expr from MongoDB to MORK.
3 - Uses load_animals_data() at morkdb_test.cc so we don't need to manually call make mork-loader FILE anymore.