memcache drivers: a long-running process will not "sense" cache-clears from other processes

[gggeek]

Imagine the following scenario:

1. process A caches item X/Y/Z => item gets saved in memcache, "curent revision" for X/, X/Y and X/Y/Z saved in both memcache and process memory. Process keeps running
2. process B invalidates X/Y. This means bumping the revision nr of X/Y in memcache
3. process A reads cache for item X/Y/Z => it takes from memory the revision number, and considers the stale item still valid

I am not sure 100% about the best way to solve this.
One way would be to add a "global counter" as memcache item, which gets bumped on every invocation of clear(). Then, it would have to always be checked in makeKeyString() before using the in-memory version of revision numbers.
A small pref loss (1 more memcache get per read/write) for more stability

[tedivm]

The new functionality automatically clears the key cache cache every second, and allows that time to be reduced and even disabled (by setting it to zero).