Add fsync.warningthresholdms property to ZooKeeperCommandExecutor

Motivation:

It is usual for fsync to take longer than 1 second in a non-prod
environment, so it'd be nice if we have an option that adjusts the
threshold.

    fsync-ing the write ahead log in SyncThread:3 took 1513ms
    which will adversely effect operation latency. File size is
    67108880 bytes. See the ZooKeeper troubleshooting guide.

Modifications:

- Set the system property `zookeeper.fsync.warningthresholdms` when a
  user specified `fsync.warningthresholdms` as an additional property.

Result:

- Closes line#366

server/src/main/java/com/linecorp/centraldogma/server/internal/replication/ZooKeeperCommandExecutor.java

@@ -331,6 +331,10 @@ private EmbeddedZooKeeper startZooKeeper() throws Exception {
             copyZkProperty(zkProps, "autopurge.snapRetainCount", "3");
             copyZkProperty(zkProps, "autopurge.purgeInterval", "1");
 
+            // Set the properties that must be set in System properties.
+            System.setProperty("zookeeper.fsync.warningthresholdms",
+                               cfg.additionalProperties().getOrDefault("fsync.warningthresholdms", "1000"));
+
             // Set the data directories.
             zkProps.setProperty("dataDir", zkDataDir.getPath());
             zkProps.setProperty("dataLogDir", zkLogDir.getPath());