Replace checkpoint_segments with min_wal_size and max_wal_size.

 Replace checkpoint_segments with min_wal_size and max_wal_size.
 
 Instead of having a single knob (checkpoint_segments) that both triggers
 checkpoints, and determines how many checkpoints to recycle, they are now
 separate concerns. There is still an internal variable called
 CheckpointSegments, which triggers checkpoints. But it no longer determines
 how many segments to recycle at a checkpoint. That is now auto-tuned by
 keeping a moving average of the distance between checkpoints (in bytes),
 and trying to keep that many segments in reserve. The advantage of this is
 that you can set max_wal_size very high, but the system won't actually
 consume that much space if there isn't any need for it. The min_wal_size
 sets a floor for that; you can effectively disable the auto-tuning behavior
 by setting min_wal_size equal to max_wal_size.
 
 The max_wal_size setting is now the actual target size of WAL at which a
 new checkpoint is triggered, instead of the distance between checkpoints.
 Previously, you could calculate the actual WAL usage with the formula
 "(2 + checkpoint_completion_target) * checkpoint_segments + 1". With this
 patch, you set the desired WAL usage with max_wal_size, and the system
 calculates the appropriate CheckpointSegments with the reverse of that
 formula. That's a lot more intuitive for administrators to set.
 
 Reviewed by Amit Kapila and Venkata Balaji N.