Support MongoDB sort by multiple fields

[pfurio]

At the moment, the datastore module of mongoDB only supports sorting documents by one field. Sorting can be done using the following keys "ASC", "ASCENDING" or "1" for ascending order; "DESC", "DESCENDING" or "-1" for descending order. We should support sorting by multiple fields allowing to pass a list of strings following this nomenclature:

field:order

Example: We want to retrieve all the documents sorted first by a priority value (1-5) in descending order and creation date in ascending order.

QueryOptions options = new QueryOptions(QueryOptions.SORT, Arrays.asList("priority:DESC", "creationDate:ASC"));