Currently, the partition index page parser will create and store
promoted index cursors for each entry. The assumption is that
partition index pages are not shared by readers so each promoted index
cursor will be used by a single index_reader (the top-level cursor).
In order to be able to share partition index entries we must make the
entries immutable and thus move the cursor outside. The promoted index
cursor is now created and owned by each index_reader. There is at most
one such active cursor per index_reader bound (lower/upper).
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