update double checked locking impl

[adriaanm]

def mkSynchronizedCheck(clazz: Symbol, cond: Tree, syncBody: List[Tree], stats: List[Tree]): Tree

is based on http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html,
but now we're on java 8 we should revisit and consult https://shipilev.net/blog/2014/safe-public-construction to make sure we're using the best variant

Should do this for both lazy vals/modules of classes and local ones.

[dwijnand]

Looking at scala/scala@d9974be it looks almost like part of the impl was changed to follow shipilev's guidance while the other was left as a TODO?