Bug 19240 (bad image list performance regression):

Bug 19240 (bad image list performance regression):
* Don't connect to the commons DB on cache hits, in order to determine the cache key name. Removed remnants of that bright idea from GlobalFunctions.php.
* Fixed total failure of negative caching in checkRedirect() due to memcached stripping trailing spaces from string values. Probably never worked.

Also:
* Respect hasSharedCache in foreign repos. Recently-added code was apparently ignorant of this setting.
* Renamed getMemcKey() to getSharedCacheKey() to make its function more clear, introduced getLocalCacheKey() to do the other thing. Fixed its parameters to be like wfMemcKey() and used it in more places.
* Used getLocalCacheKey() in various places instead of wfMemc(), to avoid having multiple repositories overwrite each others' caches.
* Fixed the BagOStuff bug that the FIXME in LocalRepo::checkRedirect() appears to refer to.
* Removed getMasterDB() and getSlaveDB() from FileRepo, it's incorrect to assume that a repo other than a LocalRepo has an associated database.
* Made FileRepo::invalidateImageRedirect() a stub, to match FileRepo::checkRedirect(). Moved the functionality to LocalRepo where checkRedirect() is concretely implemented.