Fix potential problem with btbulkdelete deleting an indexscan's current

item, if the page containing the current item is split while the indexscan
is stopped and holds no read-lock on the page.  The current item might
move right onto a page that the indexscan holds no pin on.  In the prior
code this would allow btbulkdelete to reach and possibly delete the item,
causing 'my bits moved right off the end of the world!' when the indexscan
finally resumes.  Fix by chaining read-locks to the right during
_bt_restscan and requiring btbulkdelete to LockBufferForCleanup on every
page it scans, not only those with deletable items.  Per my pghackers
message of 25-May-02.  (Too bad no one could think of a better way.)