Add logging statement for # of tasks in DNS queue

This will make DNS issues easier to debug retroactively as we will be
able to determine, by looking at the logs, if the queue size was growing
unbounded.

Also adds some logging helpers to allow programmatically choosing the level
of logging.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163123783

javatests/google/registry/dns/DnsQueueTest.java

@@ -45,7 +45,7 @@ public class DnsQueueTest {
   @Before
   public void init() {
     dnsQueue = DnsQueue.create();
-    dnsQueue.writeBatchSize = 10;
+    dnsQueue.leaseTasksBatchSize = 10;
   }
 
   @Test