Redis sess: fix path for persistent_identifier & compression_threshold

[LukeHandle]

Currently, 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. This PR removes the prefix.

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

Fixed Issues (if relevant)

Currently, persistent_identifier and compression_threshold are ignored in the Redis session configuration (the documented options for configuring).

Manual testing scenarios

Change compression_threshold to 1 (to force compression) and the sessions will not be compressed. Changing the name to be param_compression_threshold results in compression.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[LukeHandle]

Hey @maghamed, anything else needed from me here? I doubt that anyone is using the incorrectly named variants. Will also need back porting into 2.0.