Optimize iteration over the keys in the database

[xgreenx]

We have many cases when we need to iterate only over keys of the specific table/column. The current iteration logic loads both key and values into memory and later discards values. To avoid vesting of resources, we can add more functionality and provide iteration only over the keys.

As an example:
When we want to get the last block height, we load the whole CompressedBlock into the memory, when we don't need it.

[vishnuc77]

Hi @xgreenx, I'm a first time contributor trying to get involved with the project.
I would like to know whether this task is something like making use of iter_all_keys() in place of (and other similar places) :

pub fn latest_height(&self) -> StorageResult<BlockHeight> {
        self.iter_all::<FuelBlocks>(Some(IterDirection::Reverse))
            .next()
            .ok_or(not_found!("BlockHeight"))?
            .map(|(height, _)| height)
 }

so that CompressedBlock itself would not be loaded into memory, or is there something more to it?

[rymnc]

taking this up 😁

[rymnc]

should we mark this as closed with #2092?
cc: @xgreenx

[xgreenx]

Closed via #2092