Fix bufmgr so CHECKPOINT_END_OF_RECOVERY behaves as a shutdown checkpoint.

Fix bufmgr so CHECKPOINT_END_OF_RECOVERY behaves as a shutdown checkpoint.
Recovery code documents clearly that a shutdown checkpoint is executed at
end of recovery - a shutdown checkpoint WAL record is written but the buffer
manager had been altered to treat end of recovery as a normal checkpoint.
This bug exacerbates the bufmgr relpersistence bug.

Bug spotted by Andres Freund, patch by me.