Make methods private that don't need to be public.

Also: We don't need to store the buffer from the buffer manager. It is
better to access it through the new buffer() method.
Move file handling code into BufferManager.

There are new BufferManager classes calles FileInput, FileOutput and
File which contain all the code to open dump files, mmap them etc.
This was in several different places before and is now properly
encapsulated in one place.