Also use a lua script for saving cache entries (if use_lua is enabled in config)

It was the best and easiest way to solve the non-atomic way things were saved when using phpredis.
As a bonus, it even seems to perform better than the non-lua code:

2013-11-05T15:31:42+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (non-lua)...
2013-11-05T15:31:50+00:00 DEBUG (7): Page generated in 8.2447 seconds.
2013-11-05T15:31:54+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (non-lua)...
2013-11-05T15:32:03+00:00 DEBUG (7): Page generated in 8.8551 seconds.
2013-11-05T15:32:05+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (non-lua)...
2013-11-05T15:32:13+00:00 DEBUG (7): Page generated in 8.5663 seconds.
2013-11-05T15:32:14+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (non-lua)...
2013-11-05T15:32:23+00:00 DEBUG (7): Page generated in 8.7342 seconds.
2013-11-05T15:32:24+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (non-lua)...
2013-11-05T15:32:32+00:00 DEBUG (7): Page generated in 8.1831 seconds.

2013-11-05T15:32:55+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (lua)...
2013-11-05T15:33:02+00:00 DEBUG (7): Page generated in 7.0759 seconds.
2013-11-05T15:33:05+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (lua)...
2013-11-05T15:33:12+00:00 DEBUG (7): Page generated in 7.6643 seconds.
2013-11-05T15:33:14+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (lua)...
2013-11-05T15:33:22+00:00 DEBUG (7): Page generated in 7.7145 seconds.
2013-11-05T15:33:23+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (lua)...
2013-11-05T15:33:30+00:00 DEBUG (7): Page generated in 6.9632 seconds.
2013-11-05T15:33:32+00:00 DEBUG (7): Saving 10000 cache entries with 5 tags (lua)...
2013-11-05T15:33:39+00:00 DEBUG (7): Page generated in 7.1547 seconds.