Ensure all key transformations are applied by BagOStuff::makeKeyInternal()

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