distinguish balance and balance_non_root

[pereman2]

balance_non_root should be as close as possible to balance_non_root in SQLite. This commits extract balance_non_root from balance and renames balance_leaf to balance as it enables future work on a complete balance_non_root procedure.

[krishvishal]

I like this change. Because now we can just call balance which then calls the respective functions based on if its root or non_root.

Would it make sense to add a BalanceRoot variant to the WriteState enum? IMO this makes sense to add because balancing root page is a distinct operation with some edge cases and this would also help with DST.