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/allocators.cpp

@@ -24,6 +24,9 @@
 #include <unistd.h> // for sysconf
 #endif
 
+LockedPageManager* LockedPageManager::_instance = NULL;
+boost::once_flag LockedPageManager::init_flag = BOOST_ONCE_INIT;
+
 /** Determine system page size in bytes */
 static inline size_t GetSystemPageSize()
 {