Deprecate wfMemcKey() and wfGlobalCacheKey()

Deprecate wfMemcKey() and wfGlobalCacheKey()

For a while now these functions have called makeKey/makeGlobalKey() on
the default ObjectCache instances, and callers should now use those
functions directly instead.

There are other advantages to doing so, like in cases where the
ObjectCache instance being used has a different makeKey implementation
(e.g. memcache) than the default configured cache.

Change-Id: I9abf6badbf3ba800e591d2503b714433d5faf1e3