Get rid of the former rather baroque mechanism for propagating the values

of ThisStartUpID and RedoRecPtr into new backends.  It's a lot easier just
to make them all grab the values out of shared memory during startup.
This helps to decouple the postmaster from checkpoint execution, which I
need since I'm intending to let the bgwriter do it instead, and it also
fixes a bug in the Win32 port: ThisStartUpID wasn't getting propagated at
all AFAICS.  (Doesn't give me a lot of faith in the amount of testing that
port has gotten.)