Introduce a method on the Sorter to get the last inserted value of a key

[Kerollmops]

It could be useful to be able to get the latest value inserted in a sorter. There can be small lifetime issues but we can maybe just clone the value in a Vec for a first version.

The system could simply iterate on the in-memory values in revert order and then iterate over the reader in revert order until we find the requested entry. The algorithm should be O(N) for the in-memory search and O(log N) in the grenad files.