Rewrite the Directory Layer Cache implementation to use value-checks

[KrzysFR]

I tried using the \xff/metadataVersion key to implement caching in the directory layer but the experiment showed the following drawbacks:

• The DL is sensitive to outside "noise" from layers that would frequently touch the metadata version key, and requires its own key to re-validate the cache.
• The DL needs to be versioned to support this new key, which creates an issue of compatibility with other bindings.
• Even with this, the cache is discarded for unrelated changes, like creating a new subspace, or changing subspaces that were not in the cache.

Using the new Deferred Value Checks features (cf #103) we can use another approach to handle caching:

• Each subspace maintains the list of all the nodes (from the root) that were used to resolve it (ie: /Foo/Bar/Baz keeps the list of the k/v that were used to resolve Foo, then Bar, then Baz).
• Every-time a cached subspace is used in a transaction, this list of nodes will be added as deferred value-checks, but the transaction will optimistically continue (betting that nothing changed)
• Before the retry loop completes, it will resolve pending value checks. If any of the checked node was modified (by another transaction), then a fake not_committed error induce a retry.
• On the next attempt, the Directory Layer checks if there was at least one failed check using its tag, and if it was the case, discard the cache and starts over.

For the moment, we use a single tag for all the value-checks performed by the DL, so we are still discarding the whole cache even if a single subspace is changed. We may in the future be more precise?

Since we are not relying on additional keys anymore, this means that this implementation is compatible with the existing v1.0 format of the DL, and can also co-exist with scripts and programs using other bindings!