Add insert_or_replace methods

[gsserge]

Add Tree::insert_or_replace and Tree::insert_or_replace_unchecked methods. They will be used in SurrealKV configured to run without versions. The idea is that since we don't need to keep previous versions of values in the index in memory, then for the existing keys we can simply replace the previous value with the new one by using Tree::insert_or_replace. Tree::insert_or_replace_unchecked is used for loading the index, so we only keep the value with the largest version, i.e. the most recent one.

A few notes:

1. The implementation is simple but not the most optimised one. In the future we might want to add a special method for Twig nodes instead of clearing all existing values and then doing insert_mut().
2. This implementation is not sufficient for handling the online compaction with versions enabled, but let's cross that bridge when we get there. Currently the focus is to handle the case without versions.