Ensure all key transformations are applied by BagOStuff::makeKeyInternal()
Currently we have to undo any transformations we apply to keys in getMulti() by
iterating over the response array keys reversing any changes. This is hairy and
complicated. So --
* Replace calls to MemcachedBagOStuff::encodeKey() with calls to a new method,
MemcachedBagOStuff::validateKeyEncoding(). The new method only validates that
the key is compatible with memcached. If it is not, it throws an exception.
If it is, it returns the key unmodified.
* Remove MemcachedBagOStuff::{encode,decode}Key(), since they no longer serve a
purpose, and have no callers.
Change-Id: If3e20c6a1a1b42fc1f2823aa660328e37c26eccb