Fix NPE due running TestZookeeperClient

There is recurring NPE error (13x times) during running TestZookeeperClient  test case:

[INFO] Running org.apache.drill.exec.coord.zk.TestZookeeperClient 2169java.lang.NullPointerException 2170 at org.apache.zookeeper.server.persistence.FileTxnSnapLog.fastForwardFromEdits(FileTxnSnapLog.java:269) 2171 at org.apache.zookeeper.server.ZKDatabase.fastForwardDataBase(ZKDatabase.java:251) 2172 at org.apache.zookeeper.server.ZooKeeperServer.shutdown(ZooKeeperServer.java:583) 2173 at org.apache.zookeeper.server.ZooKeeperServer.shutdown(ZooKeeperServer.java:546) 2174 at org.apache.zookeeper.server.NIOServerCnxnFactory.shutdown(NIOServerCnxnFactory.java:929) 2175 at org.apache.curator.test.TestingZooKeeperMain.close(TestingZooKeeperMain.java:178) 2176 at org.apache.curator.test.TestingZooKeeperServer.stop(TestingZooKeeperServer.java:118) 2177 at org.apache.curator.test.TestingZooKeeperServer.close(TestingZooKeeperServer.java:130) 2178 at org.apache.curator.test.TestingServer.close(TestingServer.java:178) 2179 at org.apache.drill.exec.coord.zk.TestZookeeperClient.tearDown(TestZookeeperClient.java:92) 2180 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 2181 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) 2182 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) 2183 at java.lang.reflect.Method.invoke(Method.java:498) 2184 at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50) 2185 at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) 2186 at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47) 2187 at mockit.integration.junit4.JUnit4TestRunnerDecorator.invokeExplosively(JUnit4TestRunnerDecorator.java:49) 2188 at mockit.integration.junit4.FakeFrameworkMethod.invokeExplosively(FakeFrameworkMethod.java:29) 2189 at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java) 2190 at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:33) 2191 at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325) 2192 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78) 2193 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57) 2194 at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290) 2195 at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71) 2196 at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288) 2197 at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58) 2198 at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268) 2199 at org.junit.runners.ParentRunner.run(ParentRunner.java:363) 2200 at org.junit.runners.Suite.runChild(Suite.java:128) 2201 at org.junit.runners.Suite.runChild(Suite.java:27) 2202 at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290) 2203 at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71) 2204 at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288) 2205 at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58) 2206 at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268) 2207 at org.junit.runners.ParentRunner.run(ParentRunner.java:363) 2208 at org.apache.maven.surefire.junitcore.JUnitCore.run(JUnitCore.java:55) 2209 at org.apache.maven.surefire.junitcore.JUnitCoreWrapper.createRequestAndRun(JUnitCoreWrapper.java:137) 2210 at org.apache.maven.surefire.junitcore.JUnitCoreWrapper.executeLazy(JUnitCoreWrapper.java:119) 2211 at org.apache.maven.surefire.junitcore.JUnitCoreWrapper.execute(JUnitCoreWrapper.java:87) 2212 at org.apache.maven.surefire.junitcore.JUnitCoreWrapper.execute(JUnitCoreWrapper.java:75) 2213 at org.apache.maven.surefire.junitcore.JUnitCoreProvider.invoke(JUnitCoreProvider.java:158) 2214 at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:377) 2215 at org.apache.maven.surefire.booter.ForkedBooter.execute(ForkedBooter.java:138) 2216 at org.apache.maven.surefire.booter.ForkedBooter.run(ForkedBooter.java:465) 2217 at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:451)
Most likely it is needed just to update zookeper library:

• Current version: <zookeeper.version>3.5.7</zookeeper.version>
• Latest version: 3.6.2 
  _[https://mvnrepository.com/artifact/org.apache.zookeeper/zookeeper
  ]_

So current issue is minor, but updating Zookeper library can be major and easy or not easy (it depends). The separate task for updating Zookeper library can be created

The above logs can be found here (for example):
https://github.com/apache/drill/runs/1680473605?check_suite_focus=true
2168 row