Revert "Reduce checkpoints and WAL traffic on low activity database server"

Revert "Reduce checkpoints and WAL traffic on low activity database server"

This reverts commit 18fb9d8d21a28caddb72c7ffbdd7b96d52ff9724.  Per
discussion, it does not seem like a good idea to allow committed changes to
go un-checkpointed indefinitely, as could happen in a low-traffic server;
that makes us entirely reliant on the WAL stream with no redundancy that
might aid data recovery in case of disk failure.

This re-introduces the original problem of hot-standby setups generating a
small continuing stream of WAL traffic even when idle, but there are other
ways to address that without compromising crash recovery, so we'll revisit
that issue in a future release cycle.