Make recovery from WAL be restartable, by executing a checkpoint-like

Make recovery from WAL be restartable, by executing a checkpoint-like
operation every so often.  This improves the usefulness of PITR log
shipping for hot standby: formerly, if the standby server crashed, it
was necessary to restart it from the last base backup and replay all
the WAL since then.  Now it will only need to reread about the same
amount of WAL as the master server would.  The behavior might also
come in handy during a long PITR replay sequence.  Simon Riggs,
with some editorialization by Tom Lane.