The code that manages finding and removing obsolete content during checkpoint creation has grown more complex over time. It deserves being split into it's own source file and restructured a bit.

This came from some bug fixes in WT-10067.

I expect that the obsolete check code will grow more sophisticated over time, so it might be sensible to move the checkpoint sync code out from src/btree/bt_sync.c into it's own subdirectory. We can discuss whether that is warranted as part of this work during code review.