feat(tvix/store): eliminate generics in BlobStore

To construct various stores at runtime, we need to eliminate associated
types from the BlobService trait, and return Box<dyn …> instead of
specific types.

This also means we can't consume self in the close() method, so
everything we write to is put in an Option<>, and during the first close
we take from there.

Change-Id: Ia523b6ab2f2a5276f51cb5d17e81a5925bce69b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8647
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/tests/utils.rs

@@ -4,8 +4,8 @@ use crate::{
     pathinfoservice::{MemoryPathInfoService, PathInfoService},
 };
 
-pub fn gen_blob_service() -> impl BlobService + Send + Sync + Clone + 'static {
-    MemoryBlobService::default()
+pub fn gen_blob_service() -> Box<dyn BlobService> {
+    Box::new(MemoryBlobService::default())
 }
 
 pub fn gen_directory_service() -> impl DirectoryService + Send + Sync + Clone + 'static {