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

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.)