Finish work on Container_fd::open and Container_fd::close
 - add index_new_wdrop/index_closing_wdrop API for letting index
   know about data dropping accesses
 - restore metadata functions (e.g. Container::addMeta) for open/close
 - update/restore ContainerFD XAttr functions
 - stub in the old code for the rest of ContainerFD, #if0'd out.
   this is what needs to be worked on next.

related open/close comment on reference counting:
    note on revised reference counting: Container_fd can only be in one
    open mode (WRONLY, RDONLY, RDWR), and that mode won't change.  when
    a Container_fd is first opened (reference count going from 0 to 1),
    it will not have a Container_OpenFile (cof).  When the cof is
    created an index will be allocated for it using
    container_index_alloc.  Then it will be opened using index_open.
    additional references to fd will cause the cof->refcnt to be bumped
    (index_open is not called again).   when the fd is closed, the
    cof->refcnt is dropped by 1.  If it drops to zero, then the index
    is closed and removed.

    for writing, the index is notified when a new write data droping is
    created (index_new_wdop) or close (index_closing_wdrop).  the write
    droppings are identified by pid number (note tha under MPI we
    overload the pid with the MPI rank number instead).

    so the index itself doesn't have to reference count, but it does
    have to track open writing droppings so that when a write occurs
    it knows which log is getting the data (using the pid).