Namely insertion writes which over-fill the shm buffer past the latest
tsdb sample via `.tsp._history.shm_push_in_between()`.
Deats,
- check earliest `to_push` timestamp and enter pause point if it's
earlier then the tsdb's `backfill_until_dt` stamp.
- requires actually passing the `backfill_until_dt: datetime` thru,
* `get_null_segs()`
* `maybe_fill_null_segments()`
* `shm_push_in_between()` (obvi XD)