LPS-45232 UserLocalServiceImpl#getDefaultUser() isn't MVCC aware

Hey Shuyang,

This is the first MVCC issue I stumbled upon while I was helping Vilmos in finishing https://github.com/KocsisDaniel/liferay-portal/pull/377.

What I've been wondering about is whether wouldn't it be better to use MultiVMPoolUtil instead of ConcurrentHashMap in ULSI?

Thanks,
László