Move LDBM-backend specific fields of Entry struct into a private struct.

Move LDBM-backend specific fields of Entry struct into a private struct.
The new e_private field can be used for any backend purpose.  In LDBM,
it's used to point th the private entry info struct.  The LDBM entry
info struct is only visible to the LDBM cache.
WARNING: This change has not be ported to other backends!  Breakage is likely.