DRTVWR-494: Extract LockStatic as a standalone template class.

The pattern of requiring a lock to permit *any* access to a static instance of
something seems generally useful. Break out lockstatic.h; recast
LLInstanceTracker to use it.

Moving LockStatic to an external template class instead of a nested class in
LLInstanceTrackerBase leaves LLInstanceTrackerBase pretty empty. Get rid of it.

And *that* means we can move the definition of the StaticData used by each
LLInstanceTracker specialization into the class itself, rather than having to
define it beforehand in namespace LLInstanceTrackerStuff.