WIP: pre-upgrade page reservation

I attached contrib module which is base of preupgrade script. It should be part
of 8.4, but it will be required for 8.4->8.5 upgrade.

This part contains space reservation for heap/toast relations. The idea is that
relation is read and each block is checked if there is enough free space. Tuples
which will not be visible after upgrade are not count. If there is no space,
then simple_heap_update on tuple(s) until we release enough space.

BTree space reservation is more complicated. I plan to use _bt_split and split
page to two half pages with following code:

firstright = _bt_findsplitloc(rel, page, InvalidOffsetNumber, 0,&newitemonleft);
_bt_split(rel, buffer, firstright, InvalidOffsetNumber, 0, NULL,newitemonleft);
_bt_insert_parent(rel, buffer, rbuffer, stack, is_root, is_only);

Because both functions (_bt_findsplintloc, _bt_split) expect that we want to
insert new item, It will requires modification to accept InvalidOffsetNumber.

Another problem is to build stack which require to use deep tree scan. I hope
that it will not require exclusive lock on index.

I'm not yet look on hash, gist and gin. I think that hash index should be easy,
because index tuples can be moved into new bucket page. (Note: general problem
with hash index is still bitmap pages).

I guess solution for Gist index should be similar to BTree, but I don't have any
idea about GIN.

Comments, ideas, better solutions?

thanks Zdenek

PS: This patch requires previous patch which implemented space reservation
functionality.