When telling the bgwriter that we need a checkpoint because too much xlog

When telling the bgwriter that we need a checkpoint because too much xlog
has been consumed, recheck against the latest value of RedoRecPtr before
really sending the signal.  This avoids useless checkpoint activity if
XLogWrite is executed when we have a very stale local copy of RedoRecPtr.
The potential for useless checkpoint is very much worse in 8.3 because of
the walwriter process (which never does XLogInsert), so while this behavior
was intentional, it needs to be changed.  Per report from Itagaki Takahiro.