Redis sess: fix path for persistent_identifier & compression_threshold

The paths for both `persistent_identifier` & `compression_threshold` are prefixed with `param_`. This is against both the documentation and it does not follow the standard used throughout the configuration.

https://github.com/magento/devdocs/blob/develop/guides/v2.0/config-guide/redis/redis-session.md#configure-magento-to-use-redis-for-session-storage

Author: LukeHandle

lib/internal/Magento/Framework/Session/SaveHandler/Redis/Config.php

@@ -48,12 +48,12 @@ class Config implements \Cm\RedisSession\Handler\ConfigInterface
     /**
      * Configuration path for persistent identifier
      */
-    const PARAM_PERSISTENT_IDENTIFIER   = 'session/redis/param_persistent_identifier';
+    const PARAM_PERSISTENT_IDENTIFIER   = 'session/redis/persistent_identifier';
 
     /**
      * Configuration path for compression threshold
      */
-    const PARAM_COMPRESSION_THRESHOLD   = 'session/redis/param_compression_threshold';
+    const PARAM_COMPRESSION_THRESHOLD   = 'session/redis/compression_threshold';
 
     /**
      * Configuration path for compression library