Re: Recovery inconsistencies, standby much larger than primary

So here's my attempt to rewrite this logic. I ended up refactoring a
bit because I found it unnecessarily confusing having the mode
branches in several places. I think it's much clearer just having two
separate pieces of logic for RBM_NEW and the extension cases since all
they have in common is the ReadBuffer call.

I have to say, it scares the hell out of me that there are no
regression tests for this code. I'm certainly not comfortable
committing it without a few other people reading it if I haven't even
run the code once. At least I know it compiles...