We assumed page was loaded because before inserting we would move there.
`NewRowId` unfortunately moves cursor to the rightmost page causing
eviction of root page -- this arose the issue with `insert_into_page`
not loading the page we were supposed to have loaded so I added
`return_if_locked_maybe_load` which is a utility macro to check if the
page is locked and if not, load it if needed.
Closes#1138
ed1854c8de