query_result was the wrong place to put last position into. It is only
included in data-responses, but not on digest-responses. If we want to
support empty pages from replicas, both data and digest responses have
to include the last position. So hoist up the last position to the
parent structure: query::result. This is a breaking change inter-node
ABI wise, but it is fine: the current code wasn't released yet.
Closes#11072
014c5b56a3