Handling GIN incomplete splits

Here's another part of my crusade against xlog cleanup routines. This
series of patches gets rid of the gin_cleanup() function, which is
currently used to finish splits of GIN b-tree pages, if the system
crashes (or an error occurs) between splitting a page and inserting its
downlink to the parent.

The first three patches just move code around. IMHO they make the code
more readable, so they should be committed in any case. The meat is in
the fourth patch.

Thoughts, objections?

Alexander, I'm sorry if this conflicts with your GIN patches. Feel free
to post the latest versions of your patches against the current master,
ignoring patches. I can fix the bitrot. That said, I think these
refactorings will make your code look a little bit nicer too, so you
might want to rebase because of that anyway.

- Heikki