Fix longstanding race condition in transaction log management: there was a

very narrow window in which SimpleLruReadPage or SimpleLruWritePage could
think that I/O was needed when it wasn't (and indeed the buffer had already
been assigned to another page).  This would result in an Assert failure if
Asserts were enabled, and probably in silent data corruption if not.
Reported independently by Jim Nasby and Robert Creager.

I intend a more extensive fix when 8.2 development starts, but this is a
reasonably low-impact patch for the existing branches.