Take exclusive buffer lock in scan_heap() to eliminate some corner cases

Take exclusive buffer lock in scan_heap() to eliminate some corner cases
in which invalid page data could be transiently written to disk by
concurrent bgwriter activity.  There doesn't seem any risk of loss of
actual user data, but an empty page could possibly be left corrupt if a
crash occurs before the correct data gets written out.  Pointed out by
Alvaro Herrera.