Change checkpoint_completion_target default to 0.9

Change checkpoint_completion_target default to 0.9

Common recommendations are that the checkpoint should be spread out as
much as possible, provided we avoid having it take too long.  This
change updates the default to 0.9 (from 0.5) to match that
recommendation.

There was some debate about possibly removing the option entirely but it
seems there may be some corner-cases where having it set much lower to
try to force the checkpoint to be as fast as possible could result in
fewer periods of time of reduced performance due to kernel flushing.
General agreement is that the "spread more" is the preferred approach
though and those who need to tune away from that value are much less
common.

Reviewed-By: Michael Paquier, Peter Eisentraut, Tom Lane, David Steele,
Nathan Bossart
Discussion: https://postgr.es/m/20201207175329.GM16415%40tamriel.snowman.net