Following on from WT-16641's findings, we should lift up the b-tree size computation into the b-tree layer if possible. This will mean that the block manager no longer owns size tracking which will be a nice improvement.

Scope:

• Investigate if this is indeed possible, if we do not have the relevant block metadata at the b-tree layer then we will need to continue with the current implementation. Regardless the second piece of this ticket.
• Define 2 functions, b-tree size increase, b-tree size decrease. Hook into the relevant paths, either up in b-tree or down in block manager.
• Test