incremental_reader_selector: don't jump over sstables
incremental_reader_selector: don't jump over sstables

Passing the current read position to the
`incremental_selector::select()` can lead to "jumping" through sstables.
This can happen when the currently open sstables have no partition that
intersects with a yet unselected sstable that has an intersecting range
nevertheless, in other words there is a gap in the selected sstables
that this unselected one completely fits into. In this case the
unselected sstable will be completely omitted from the read.
The solution is to not to avoid calling `select()` with a position that
is larger than the `next_position` returned from the previous `select()`
call. Instead, call `select()` repeatedly with the `next_position` from
the previous call, until either at least one new sstable is selected or
the current read position is surpassed. This guarantess that no sstables
will be jumped over. In other words, advance the incremental selector in
a pace defined by itself thus guaranteeing that no sstable will be
jumped over.

scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla