Support reading/writing RocksDB incremental checkpoints and make formats explicit

[gyfora]

Currently only rocksdb savepoint formats are supported which is a silent assumption not explicitly checked and enforced in many places where we could.

We should extend support for RocksDB incremental checkpoint reading and writing as that is a very common use-case.

[gyfora]

There is now support for reading, writing may not be so important atm