* Second try of split message caches with changes suggested by Tim Starling and other...

* Second try of split message caches with changes suggested by Tim Starling and other small fixes
 * removed use of umask, wfMkdirParents works now
 * splitting of messages per language is now unconditional
 * storing cached messages in 2-d array of code => messages
 * fixed a bug in replace(), now deletion works properly
 * clear() should actually do something now
 * try to delay parser creation in transform()
 * removed the use of $memcPrefix and wfMemcKeyLang() in favour of wfMemcKey()
 * escape $code in loadFromDb()
 * use member variable instead of static variable for tracking loaded languages