Hierarchy: Use $this->backendCount

No point in doing a count on the backends array if we already have the
count accessible.
Hierarchy: Mark as experimental
KeyChange: Update getMultiple variable names

The keys aren't necessarily prefixed because it's up to the derived
class to change them in whatever way they want.
KeyShorten: Validate max length

Otherwise weird things happen when we try to shorten keys because of a
negative value to substr.
Local: Don't assume that Memcache is used

Just in a comment. It wasn't really accurate how it was before.
KeyChange: Add changeKeys method

This method allows changing multiple keys at once. Purely for
performance reasons to avoid multiple calls to changeKey which may
invoke lots of calls to their own methods.