sstable: clean up two/single level iterator init

[RaduBerinde]

The twoLevelIterator initialization code repeats most of the singleLevelIterator initialization code. Sometimes the two code paths unintentionally diverge (e.g. in 23.2/24.1 one used a preallocated read handle and one didn't).

This issue tracks cleaning up this initialization code - the common parts should be extracted out and reused. The singleLevelIterator fields should be initialized from the single level iterator code, not directly from the two-level code.

Jira issue: PEBBLE-34

[RaduBerinde]

Fixed by #3738.