refactor(tvix/castore/directory/grpc_wrapper): use ClosureValidator

This greatly simplifies the code in this function, replacing it with a
much better tested (and more capable!) version of the validation logic.

It also enables the gRPC server frontend to make use of the
DirectoryPutter interface. While this might not be too visible in terms
of latency thanks to gRPC streams bursting, it also enables further
optimizations later (such as bucketing of directory closures).

Change-Id: I21f805aa72377dd5266de3b525905d9f445337d6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11221
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/castore/src/directoryservice/closure_validator.rs

@@ -39,7 +39,7 @@ pub struct ClosureValidator {
 impl ClosureValidator {
     /// Insert a new Directory into the closure.
     /// Perform individual Directory validation, validation of insertion order
-    // and size fields.
+    /// and size fields.
     #[instrument(level = "trace", skip_all, fields(directory.digest=%directory.digest()), err)]
     pub fn add(&mut self, directory: proto::Directory) -> Result<(), Error> {
         let digest = directory.digest();