freezer: implement split files for data

* freezer: implement split files for data

* freezer: add tests

* freezer: close old head-file when opening next

* freezer: fix truncation

* freezer: more testing around close/open

* rawdb/freezer: address review concerns

* freezer: fix minor review concerns

* freezer: fix remaining concerns + testcases around truncation

* freezer: docs

* freezer: implement multithreading

* core/rawdb: fix freezer nitpicks + change offsets to uint32

* freezer: preopen files, simplify lock constructs

* freezer: delete files during truncation