add code for ContainerFD::open and adjust framework as needed:
 - new ContainerIndex callback "index_new_wdrop" called when a new
   write data dropping is created (so the index knows about it)

 - add ContainerOpenFile.h with data structures for open files.
   this has all the non-index related data from the old Container_OpenFile,
   Index, and WriteFile merged into one place.

 - pull in all the old code used to create subdirs into tree... including
   handling of shadow containers, Metalinks, etc. and match it to the
   new framework.   also code to handle delayed creation of write
   droppings (delay until first write).

   (ignore the "#if 0"'d chunk of code at the end of
    Container_fd::establish_helper... some minor bits here that need
    to be pulled in... mainly the call to Container:;addOpenrecord needs
    to be added back still)