Changing LockedPageManager to use a managed instance

This ensures the allocator is ready no matter when it's needed (as
some STL implementations allocate in constructors -- i.e., MSVC's STL
in debug builds).

Using boost::call_once to guarantee thread-safe static initialization.

Adding some comments describing why the change was made.

Addressing deinitialization of the LockedPageManager object
by initializing it in a local static initializer and adding
an assert in the base's destructor.

src/util.cpp

@@ -95,8 +95,6 @@ void locking_callback(int mode, int i, const char* file, int line)
     }
 }
 
-LockedPageManager LockedPageManager::instance;
-
 // Init
 class CInit
 {