Storage Plugins Handler improvements

Storage plugin handler can be improved with:

• Incremental update of storage plugins configs. For example in case of Hive, oldPluginConfig may have a key ("hive.metastore.sasl.enabled") with value true in configProps whereas newPluginConfig doesn't have this key at all. In that case if Incremental Update is enabled the old value for the property should remain.
• Versioning can be implemented by introducing timestamp:long in the DataChangeVersion.
  Zookeper ZNode stats has timestamp values. So timestamp when storage-plugins.conf file was created can be used in the process of updating plugin. When timestamp of file and plugin config from PersistentStore is the same - nothing need to do
• The approach of updating plugins configs without restarting drillbit could be implemented.
• It can be extracted to the separate service, like Installer Agent, which can generate storage-plugins.conf files depends on the environment and system env properties.