ZOOKEEPER-3385: fix build break.

PR https://github.com/apache/zookeeper/pull/939 that merged in master caused build break due to
two missing imports. Fix the build break by adding the missing imports.

Author: Michael Han <lhan@twitter.com>

Reviewers: Enrico Olivelli <eolivelli@apache.org>, Norbert Kalmar <nkalmar@apache.org>

Closes #967 from hanm/ZOOKEEPER-3385

zookeeper-server/src/main/java/org/apache/zookeeper/server/admin/Commands.java

@@ -35,15 +35,13 @@ import org.apache.zookeeper.Version;
 import org.apache.zookeeper.server.DataTree;
 import org.apache.zookeeper.server.ServerCnxnFactory;
 import org.apache.zookeeper.server.ServerMetrics;
-import org.apache.zookeeper.server.ServerStats;
 import org.apache.zookeeper.server.ZooKeeperServer;
 import org.apache.zookeeper.server.ZooTrace;
 import org.apache.zookeeper.server.persistence.SnapshotInfo;
-import org.apache.zookeeper.server.quorum.Follower;
-import org.apache.zookeeper.server.quorum.FollowerZooKeeperServer;
 import org.apache.zookeeper.server.quorum.Leader;
 import org.apache.zookeeper.server.quorum.LeaderZooKeeperServer;
-import org.apache.zookeeper.server.quorum.ObserverZooKeeperServer;
+import org.apache.zookeeper.server.quorum.QuorumPeer;
+import org.apache.zookeeper.server.quorum.QuorumZooKeeperServer;
 import org.apache.zookeeper.server.quorum.ReadOnlyZooKeeperServer;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;