store: internal locking, remove Arc<Mutex<Store>> wrapper

Store now has internal Mutex for capnp appends and AtomicU64 for
size tracking. All methods take &self. The external Arc<Mutex<Store>>
is replaced with Arc<Store>.

- Store::append_lock protects file appends
- local.rs functions take &Store (not &mut Store)
- access_local() returns Arc<Store>
- All .lock().await calls removed from callers

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/hippocampus/memory.rs

@@ -19,8 +19,7 @@ pub struct MemoryNode {
 impl MemoryNode {
     /// Load a node from the store by key.
     pub fn load(key: &str) -> Option<Self> {
-        let arc = super::access_local().ok()?;
-        let store = arc.try_lock().ok()?;
+        let store = super::access_local().ok()?;
         Self::from_store(&store, key)
     }