Fixed incorrect continuation token when searching with sort

The continuation token was generated based on _mongo.partitionKey.
This is correct when _mongo.partitionKey is the only sort field.  When
there are other sort fields in the query, the sequence of records are
influenced by those fields. Continuation token based on only
_mongo.partitionKey value does not produce the correct results for next
pages.

There is a npm package on "cursor based pagination":
https://www.npmjs.com/package/mongo-cursor-pagination
The library supports one "paginatedField".  The ClearlyDefined api
supports sort resulting in multiple sort fields. For example: sort on
namespace sorts on three fields: ['coordinates.namespace',
'coordinates.name', 'coordinates.revision'].  So the library is not
sufficient for CD's need.

To generate the correct next page, the continuation token needs to
record all the values for sort fields. As an extension of existing logic,
_mongo.partitionKey will always be included to the sort fields as a tie
breaker. Also added logic to generate pagination query based on
multiple sort values.