Cache misses on Redis backend

[crsantini]

Prerequisites:

• W3TC version: 0.9.6.1
• PHP-FPM 7.2
• WP version 4.9.6
• Redis

We have issue with incorrect cache misses for cached pages on Redis backend.
Steps to reproduce:

1. Redis-cli: Enable MONITOR mode
2. load already cached page via browser

Actual result: we got two calls - the first has correct cached page key but the second one is "w3tc_1_pgcache__722264618_key_version" and this key name doesn't change for any other cached page