In preparation for supporting more than one algorithm for lookups in
the promoted index, extract relevant logic out of the index_reader
(which is a partition index cursor).
The clustered index cursor implementation is now hidden behind
abstract interface called clustered_index_cursor.
The current implementation is put into the
scanning_clustered_index_cursor. It's mostly code movement with minor
adjustments.
In order to encapsulate iteration over promoted index entries,
clustered_index_cursor::next_entry() was introduced.
No change in behavior intended in this patch.
d5bf540079