Use locking on async mapreduces

This ensures that only one will run at a time, which should help fix the
clogged up mapreduces we've seen on sandbox.

In order to do this, the UnlockerOutput is introduced. This unlocks the
given Lock after all reducer shards have finished.

Also increases the lease duration of the DNS refresh action from 20 to
240 minutes. 20 minutes isn't long enough; when there's a lot of domains
and decent system load the mapreduce could take longer than that in the
ordinary case.

TESTED=Deployed to alpha and verified that more than one copy of the
mapreduce wouldn't run simultaneously, and also that the lock is
released when the mapreduce is finished.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=205887554

java/google/registry/request/lock/LockHandlerImpl.java

@@ -105,7 +105,7 @@ public class LockHandlerImpl implements LockHandler {
   /** Allows injection of mock Lock in tests. */
   @VisibleForTesting
   Optional<Lock> acquire(String lockName, @Nullable String tld, Duration leaseLength) {
-    return Lock.acquire(lockName, tld, leaseLength, requestStatusChecker);
+    return Lock.acquire(lockName, tld, leaseLength, requestStatusChecker, true);
   }
 
   /** A {@link Callable} that acquires and releases a lock around a delegate {@link Callable}. */