WL#2724: Diskdata: Varsize

Impl. varsize records for diskdata. This has 3 phases:
    * Phase 1: varsize every where except in TUP (WL 4499)
    * Phase 2: Change TUP DD format to be *dynamic* ONLY, storage will
still be of fixed size (WL 4569)
    * Phase 3: Varsize data will be stored as varsize on disk page (wl#4755,
wl#4788, wl#4817, wl#4919, wl#4920)
.

- *all* DD columns will be stored as "dynamic" (wl4569)
-  other parts of system should be unchanged by this change (i.e it
            should *only* be in TUP)

WL 2724: Steps:
- Change the space alloc'ing for a record to varsize. Today, a disk page 'knows'
the size of the page and record, so it 'knows' how many records can be
allocated. Change this mechanism.
- Infra structure changes for retriving pages from the extent wl#4755, finished
- Ensure that varsz works for insert/delete (finished wl#4788)
- Ensure that update works (wl#4817, finished)
- Change logging to varsize (wl#4919, finished)
- Ensure that recovery works (wl#4920, finished)