Restructure smgr API as per recent proposal. smgr no longer depends on

Restructure smgr API as per recent proposal.  smgr no longer depends on
the relcache, and so the notion of 'blind write' is gone.  This should
improve efficiency in bgwriter and background checkpoint processes.
Internal restructuring in md.c to remove the not-very-useful array of
MdfdVec objects --- might as well just use pointers.
Also remove the long-dead 'persistent main memory' storage manager (mm.c),
since it seems quite unlikely to ever get resurrected.