Add `makeKey` and `makeGlobalKey` to BagOStuff

Add `makeKey` and `makeGlobalKey` to BagOStuff

* Add a string `keyspace` member to BagOStuff instances. The default
  implementation, meant for simple key/value stores, treats the key space
  as a string prefix to prepend to keys. By default, its value is `local`,
  but any instance created via ObjectCache::newFromParams() (or or one of
  its callers) will have that default to $wgCachePrefix / wfWikiID().
* Add `makeKey` and `makeGlobalKey` methods to the base BagOStuff class.
  These methods are not static to allow for BagOStuff types which require
  a configured instance to know the underlying storage engine's key semantics.
* Make wfMemcKey() and wfGlobalCacheKey() delegate to these methods on the main
  ObjectCache instance.

Change-Id: Ib7fc2f939be3decfa97f66af8c2431c51039905f