Fix an issue in recent walwriter hibernation patch.

Fix an issue in recent walwriter hibernation patch.

Users of asynchronous-commit mode expect there to be a guaranteed maximum
delay before an async commit's WAL records get flushed to disk.  The
original version of the walwriter hibernation patch broke that.  Add an
extra shared-memory flag to allow async commits to kick the walwriter out
of hibernation mode, without adding any noticeable overhead in cases where
no action is needed.