block_util: Avoid intermediate completion queue allocation

Rather than aggregate the completion list into an intermediate vector
instead adjust the API to provide one completion item at a time.

With DHAT this shows the number of heap allocations has decreased.

Before:

    dhat: Total:     623,852 bytes in 8,157 blocks

After:

    dhat: Total:     380,444 bytes in 3,469 blocks

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

block_util/src/raw_sync.rs

@@ -127,7 +127,7 @@ impl AsyncIo for RawFileSync {
         Ok(())
     }
 
-    fn complete(&mut self) -> Vec<(u64, i32)> {
-        self.completion_list.drain(..).collect()
+    fn next_completed_request(&mut self) -> Option<(u64, i32)> {
+        self.completion_list.pop()
     }
 }