Buffer manager modifications to keep a local buffer-dirtied bit as well

Buffer manager modifications to keep a local buffer-dirtied bit as well
as a shared dirtybit for each shared buffer.  The shared dirtybit still
controls writing the buffer, but the local bit controls whether we need
to fsync the buffer's file.  This arrangement fixes a bug that allowed
some required fsyncs to be missed, and should improve performance as well.
For more info see my post of same date on pghackers.