MAINT-4952: Use IntrusivePtr for BufferArray,HttpHeaders,HttpOptions.

Specifically, change the ptr_t typedefs for these LLCore classes to use
IntrusivePtr rather than directly using boost::intrusive_ptr. This allows us
to use a simple ptr_t(raw ptr) constructor rather than having to remember to
code ptr_t(raw ptr, false) everywhere. In fact, the latter form is now invalid:
remove the now-extraneous 'false' constructor parameters.