memory: reuse freed nodes in WriteLog

Once a node is freed it can be used in next allocation.  Change WriteLog
such that it tries to pick freed nodes first before calling out to the
allocator when customer tries to allocate a new node.

common/memory/src/lib.rs

@@ -195,9 +195,14 @@ impl<'a, A: Allocator> WriteLog<'a, A> {
     pub fn allocator(&self) -> &A { &*self.alloc }
 
     pub fn alloc(&mut self, value: A::Value) -> Result<Ptr, OutOfMemory> {
-        let ptr = self.alloc.alloc(value)?;
-        self.allocated.push(ptr);
-        Ok(ptr)
+        Ok(if let Some(ptr) = self.freed.pop() {
+            self.set(ptr, value);
+            ptr
+        } else {
+            let ptr = self.alloc.alloc(value)?;
+            self.allocated.push(ptr);
+            ptr
+        })
     }
 
     pub fn set(&mut self, ptr: Ptr, value: A::Value) {