Revert (but cleanup) SingletonHolder implementation

MSVC 2017 15.7 contains a bug where constant initialization no longer
takes place for objects with constructors marked constexpr such as
once_flag. This leaves once_flag susceptible to dynamic initialization
order problems and causes some singleton instances to be constructed
incorrectly. We put back our correct implementation (with some cleanup)
and await a fix from MS.
Refs #0