Restructure entity caching so it can be cleared more easily

Memcache itself has no built-in support for wildcard flushes. Both memcache and memcache_storage modules get around this by keeping track of each time cache_clear_all() is called with a wildcard pattern, incrementing a counter for the pattern being flushed, and using that count in a hash of the $cid value. Incrementing the count has the effect of changing the CID used for entries that match the wildcard pattern, causing new values to be cached/retrieved for those entries.

This works well. The problem is that both modules store the wildcard flush counts in the variable table. Since metatag module can execute a wildcard flush for just about every entity on the site, this variable can quickly become bloated. Right now on my site, this variable value is 4 MB long and takes 6 seconds to update.

This is probably a "won't fix" for Metatag. I still thought it was worth mentioning in the issue queue, since Memcache is extremely popular.