BUG #3674: Unnecessary checkpoints by WAL Writer

The following bug has been logged online:

Bug reference:      3674
Logged by:          ITAGAKI Takahiro
Email address:      itagaki.takahiro@gmail.com
PostgreSQL version: 8.3beta1
Operating system:   independent
Description:        Unnecessary checkpoints by WAL Writer
Details:

WAL Writer requests unnecessary checkpoints with CHECKPOINT_CAUSE_XLOG.
RedoRecPtr, declared in xlog.c, is initialized at StartupXLOG() but never
updated in WAL Writer because it never calls XLogInsert(). It judges excess
of segments wrongly.

Though bgwriter also does not call XLogInsert(), but it would be OK because
bgwriter updates RedoRecPtr in CreateCheckPoint().