[SPARK-18557] Downgrade confusing memory leak warning message

## What changes were proposed in this pull request?
TaskMemoryManager has a memory leak detector that gets called at task completion callback and checks whether any memory has not been released. If they are not released by the time the callback is invoked, TaskMemoryManager releases them.

The current error message says something like the following:
```
WARN  [Executor task launch worker-0]
org.apache.spark.memory.TaskMemoryManager - leak 16.3 MB memory from
org.apache.spark.unsafe.map.BytesToBytesMap33fb6a15
In practice, there are multiple reasons why these can be triggered in the normal code path (e.g. limit, or task failures), and the fact that these messages are log means the "leak" is fixed by TaskMemoryManager.
```

To not confuse users, this patch downgrade the message from warning to debug level, and avoids using the word "leak" since it is not actually a leak.

## How was this patch tested?
N/A - this is a simple logging improvement.

Author: Reynold Xin <rxin@databricks.com>

Closes #15989 from rxin/SPARK-18557.

core/src/main/java/org/apache/spark/memory/TaskMemoryManager.java

@@ -378,14 +378,14 @@ public class TaskMemoryManager {
       for (MemoryConsumer c: consumers) {
         if (c != null && c.getUsed() > 0) {
           // In case of failed task, it's normal to see leaked memory
-          logger.warn("leak " + Utils.bytesToString(c.getUsed()) + " memory from " + c);
+          logger.debug("unreleased " + Utils.bytesToString(c.getUsed()) + " memory from " + c);
         }
       }
       consumers.clear();
 
       for (MemoryBlock page : pageTable) {
         if (page != null) {
-          logger.warn("leak a page: " + page + " in task " + taskAttemptId);
+          logger.debug("unreleased page: " + page + " in task " + taskAttemptId);
           memoryManager.tungstenMemoryAllocator().free(page);
         }
       }