Incorrect double-checked locking idiom

[olotenko]

https://github.com/oracle/helidon/blob/1dacd76/common/context/src/main/java/io/helidon/common/context/ListContext.java#L190-L191 - the order of assignments allows to observe instance to be null.

The correct order is:

        private T instance; // no need to declare it volatile
        public T get() {
            if (missing) {
                synchronized (this) {
                    if (missing) {
                        instance = supplier.get();
                        missing = false; // publish only after a valid value is assigned to instance
                    }
                }
            }
            return instance; // missing is volatile, so this read is going to happen
        }

Also, you probably do not need to hold on to supplier after the first use.
ListContext.java.1dacd76.diff.zip - a diff against 1dacd76

[tomas-langer]

@olotenko could you please have a look at the PR?
I have added a slightly more complex fix - I have created a LazyValue interface that is used to wrap the concept of "get once and cache" and to be properly thread safe (without using the synchronized keyword.
I have also updated our ExecutorService suppliers to use the new component.