Refactor cache_clear_all() behaviour

#1103478: Skip some wildcard fetches attempts to reduce the number of memcache gets for wildcards. To do this, it has to always rely on the variable system to register full bin flushes (currently this only happens if you have minimum cache lifetime enabled).

While in testing this removes a memcache get from normal page caching, and should reduce the number of keys fetched via getMulti a fair bit overall, it means that every node and comment save triggers a variable_set(), which is an issue in itself. If you use something like strongarm module, variable_set() can trigger a full second or two of processing, and in D7 it's a read lock across the entire site for variable cache misses.

I'd like to refactor the current code, so that it allows modules to override the behaviour of cache_clear_all() - we could then ship with a default implementation (in memcache module or another extra module), or people could use http://drupal.org/project/expires instead.

One idea would be to set a variable in the future (say minimum_cache_lifetime * .8) - this would set a timestamp when the cache is going to actually be cleared (as well as the session check). Until that timestamp == time(), if another process clears the cache, we check the existing variable, and if it's still in the future, we don't bother updating it. Once the timestamp is hit, the cache is actually cleared, and the current minimum cache lifetime logic kicks in for the remaining .2 of minimum_cache_lifetime() (so you can still avoid a stampede - and memcache has its own stampede logic anyway).

Say you have 50 comments posted per hour, and a minimum cache lifetime of an hour, this would mean at most two variable_set() per hour instead of 50. It's not quite the same behaviour as the actual minimum cache lifetime, but I don't there's a good way to do this that will actually work for all sites, so making it hookable with the minimum scaffolding in memcache.inc seems like the best bet.