## Problem
We currently clone `WriteState` in every loop iteration of
`insert_into_page()`, which was probably done for borrow checker
reasons, but since `WriteState` has expanded to include buffers that
must not be moved in memory or dropped, it has necessitated a really
annoying workaround of wrapping the buffers in `Arc<Mutex>>` which is
just completely wasteful.
## Fix
Do not clone `WriteState` in `insert_into_page()`, and instead work with
the borrow checker a bit more. Note that `WriteState` still _implements_
`Clone` because it's also cloned in `balance_non_root()` - that can be a
separate refactor.
Reviewed-by: Avinash Sajjanshetty (@avinassh)
Reviewed-by: Nikita Sivukhin (@sivukhin)
Closes#2464