Make a more efficient update walker

[johnjamesjacoby]

Over the years, the new/update forum/topic/reply code has accidentally become a mish-mash of approaches to address specific needs at the time. This has resulted in multiple approaches to walking & traversing the post_parent hierarchy, making it not a very straightforward process to understand or hook into.

A few things to consider:

• Our handler functions should be the only places where user input is checked
• Our walker function should be the only places where that branch of the tree is updated
• Our extras and update functions should be helpers, but not invoke a walk of the tree or handle user input
• Some functions require a recalculation of the tree, like spamming & unspamming a reply. In these cases, the walkers should know how to query for and update the relevant meta-data
• Caching should be scrutinized deeply to make sure we aren't triggering multiple recalculations per iteration of the walker