Ensure backwards and descending pagination work with id-only optimization.

[myronmarston]

In #205, I implemented an optimization, where we avoid a datastore query when the client is only requesting ids which we already have. As part of that, I implemented sorting and pagination support so that the expected page and ordering of ids are returned.

However, I only implemented the simplest case: foward ascending pagination. After merging that, I realized that my implementation wouldn't work for the three other pagination cases:

• Forward descending pagination (e.g. first: N, after: cursor, orderBy: [id_DESC])
• Backwards ascending pagination (e.g. last: N, before: cursor, orderBy: [id_ASC])
• Backwards descending pagination (e.g. last: N, before: cursor, orderBy: [id_DESC])

To fix those three cases, I had to change two things in the implementation:

• Get the sort from the pagination datastore body rather than directly from query. query.sort contains the sort order requested in the GraphQL query, but for some of the above cases we wind up reversing the sort direction we send in the datastore query.
• Apply the pagination cursor correctly when results are sorted descending. Previously, we filtered using id > search_after for all cases, but we need to use id < search_after when we are sorting descending.

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[BrianSigafoos-SQ]

LGTM