Database keys are not sorted correctly.

## Bug

integer keys in the database are sorted lexicographically, which is not how they should be stored.

### Expected behavior
Integer keys should be sorted by value.

## Steps to reproduce

Increase the range here and it will fail:

https://github.com/Cuprate/cuprate/blob/b76042a4e4f364aa2c3b53235f30382ac632d2e8/storage/cuprate-blockchain/src/backend/tests.rs#L336-L340
