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