arenaskl: consider alternative TrySeekingUsingNext optimizations

The arenaskl Iterator implementation implements the TrySeekUsingNext optimization through performing simple Nexts:

https://github.com/cockroachdb/pebble/blob/6c6fd7553fc59a1223ff0289f3c02164c23d442e/internal/arenaskl/iterator.go#L94-L115

If it doesn't find the sought key within 5 nexts, it abandons the work it's already done and starts anew. We could instead take advantage of the skiplist node structure and try advancing at levels above the base level but below the list height.

I'm imagining something like:
```
Initialize level = 0
for range 5 {
next := n.tower[level]
if key <= next.key {
return searchDescending(next, level, key)
}
n = next
if n.height > level+1 {
level++
}
}
```

Jira issue: PEBBLE-1197