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/nar_renderer.rs

@@ -1,5 +1,3 @@
-use crate::blobservice::BlobService;
-use crate::blobservice::BlobWriter;
 use crate::directoryservice::DirectoryService;
 use crate::nar::NARRenderer;
 use crate::proto::DirectoryNode;