Optimize large append.

This commit fixes an issue where large nodes would take up most of the insert time because the entire node size would be calculated to check if it could fit in a page or not. This changes the behavior
so that a node's size is only calculated up to the size it needs to check and then returns.

See `node.sizeLessThan()` below.

/cc @snormore @mkobetic
This function does the same thing as `size()` except that it checks after each inode if the size is above the threshold and then escapes.