Fix for Lua script when removing a huge amount of keys in _removeByMatchingAnyTags()

Hi Colin,

I'm using ```Cm_Cache_Backend_Redis``` on a large Magento shop which easily generates up to 1.000.000+ keys within a few hours.

~~Recently, when refreshing for example the Block Html cache, we've experienced an error:~~
When building a huge array of tags (>8000) that need to be removed from Redis, that happens usually when re-indexing, the following error occurs:
```
too many results to unpack
```

Lua's ```unpack()``` has a limit on the size of the table imposed by the number of Lua stack slots that a C function can use. This value is defined by LUAI_MAXCSTACK in luaconf.h and for Redis it is set to 8000.

So I have added a small change to the Lua script in the ```_removeByMatchingAnyTags()``` method to process the Redis calls in 5000 keys steps.

Please take a look and see if it makes sense. Maybe you have a better approach?

Thanks!