Make pcoinsTip memory calculations consistent

Since we are more accurately measuring pcoinsTip peak usage at twice the current in dynamic usage, it makes sense to double the default (this will lead to the same effective usage and peak usage as previously).
We should also double the buffer used to avoid flushing if above 90% but still sufficient space remaining.

Github-Pull: #10133
Rebased-From: 5b95a190e8d7059039ce61e808d494dcf89ebb3b