Fast promotion failure

While testing the bug from the "Assertion failure at standby promotion",
I bumped into a different bug in fast promotion. When the first
checkpoint after fast promotion is performed, there is no guarantee that
the checkpointer process is running with the correct, new,
ThisTimeLineID. In CreateCheckPoint(), we have this:

> /*
> * An end-of-recovery checkpoint is created before anyone is allowed to
> * write WAL. To allow us to write the checkpoint record, temporarily
> * enable XLogInsertAllowed. (This also ensures ThisTimeLineID is
> * initialized, which we need here and in AdvanceXLInsertBuffer.)
> */
> if (flags & CHECKPOINT_END_OF_RECOVERY)
> LocalSetXLogInsertAllowed();

That ensures that ThisTimeLineID is updated when performing an
end-of-recovery checkpoint, but it doesn't get executed with fast
promotion. The consequence is that the checkpoint is created with the
old timeline, and subsequent recovery from it will fail.

I ran into this with the attached script. It sets up a master (M), a
standby (B), and a cascading standby (C). I'm not sure why, but when I
tried to simplify the script by removing the cascading standby, it
started to work. The bug occurs in standby B, so I'm not sure why the
presence of the cascading standby makes any difference. Maybe it just
affects the timing.

- Heikki