Add GetNearest method to the DB interface

[babadro]

Summary

There is an issue in cosmos-sdk: Make KVStore interface have methods to getNearest entry.

To address this issue, we can add a new method, GetNearest to the DB interface and implement it for all supported storage engines. This method should have a similar cost to the Get operation and provide a more efficient alternative to iterators for finding the nearest key.

// GetNearest retrieves the nearest key to the given key.
// If 'ascending' is true, the method returns the smallest key greater than the given key.
// If 'ascending' is false, the method returns the largest key smaller than the given key.
// In case there is no key in the desired direction, the method returns nil.
// CONTRACT: key, value read-only []byte
GetNearest(key []byte, ascending bool) ([]byte, error)

Once implemented, the method can be used in the cosmos-sdk repo to add a GetNearest function to the KVStore interface, as described in the original issue.

[elias-orijtech]

@babadro are you actively working on this?

[babadro]

@elias-orijtech
Not yet.
I was uncertain whether I correctly understood the concept and if the problem is still relevant considering the age of the original issue.
Hence, I decided to verify it by opening this discussion first.

[ValarDragon]

I still believe that this would be valuable.

A very significant percentage of iterator usage in Osmosis is this use case. (And suffers kind of insane overheads)