refactor and cleanup [skip ci]

submodules/Postbox/Sources/CachedItemView.swift

@@ -0,0 +1,31 @@
+import Foundation
+
+final class MutableCachedItemView: MutablePostboxView {
+    private let id: ItemCacheEntryId
+    fileprivate var value: PostboxCoding?
+    
+    init(postbox: Postbox, id: ItemCacheEntryId) {
+        self.id = id
+        self.value = postbox.itemCacheTable.retrieve(id: id, metaTable: postbox.itemCacheMetaTable)
+    }
+    
+    func replay(postbox: Postbox, transaction: PostboxTransaction) -> Bool {
+        if transaction.updatedCacheEntryKeys.contains(self.id) {
+            self.value = postbox.itemCacheTable.retrieve(id: id, metaTable: postbox.itemCacheMetaTable)
+            return true
+        }
+        return false
+    }
+    
+    func immutableView() -> PostboxView {
+        return CachedItemView(self)
+    }
+}
+
+public final class CachedItemView: PostboxView {
+    public let value: PostboxCoding?
+    
+    init(_ view: MutableCachedItemView) {
+        self.value = view.value
+    }
+}