public MonitoredTask get() {
      return impl;
    }
This class encapsulates an object as well as a weak reference to a proxy that passes through
calls to that object. In art form:

<pre>
    Proxy  <------------------
      |                       \
      v                        \
PassthroughInvocationHandler   |  weak reference
      |                       /
MonitoredTaskImpl            /
      |                     /
StatAndWeakRefProxy  ------/
</pre>

Since we only return the Proxy to the creator of the MonitorableStatus, this means that they
can leak that object, and we'll detect it since our weak reference will go null. But, we still
have the actual object, so we can log it and display it as a leaked (incomplete) action.
hbase-server/src/main/java/org/apache/hadoop/hbase/monitoring/TaskMonitor.java