Use same Iterator for ChainDB as for the ImmutableDB

After input-output-hk/ouroboros-network#2534, we could use the same `Iterator` and `IteratorResult` records for both the ChainDB and the ImmutableDB.

The remaining differences are:
* In ImmutableDB but not in ChainDB:
```haskell
iteratorHasNext :: HasCallStack => STM m (Maybe (RealPoint blk))
```
I think we can just add it to the ChainDB.
* The ChainDB's `IteratorResult` has the following constructor that the ImmutableDB's doesn't have:
```haskell
IteratorBlockGCed (RealPoint blk)
```
We can instantiate `IteratorResult` with `Either (BlockGCed blk) b` instead of `b` for the ChainDB.