Revert "another attempt to convert PageBuilder.Write to use async.  this attempt is faster than the previous one, but still 70% slower than just blocking on synchronous writes.  this attempt converts both LeafState and ParentState to use mutables, then a simple loop over the items instead of using Seq/List.fold.  I also made some effort to make sure asyncs don't get created for cases where they are not needed, such as adding a non-overflowing kvp to a leaf.  still kinda slow."

This reverts commit 49b83db22208526a319c82fe5de5d01691a915e5.