Allow scanning partition backwards

[albe]

Currently, partitions can only be scanned forward from a known position (0 without a persisted index), because the data block size is only stored in the document header/prefix.
This makes things simpler in the partition, but strictly requires a consistent index in order to allow random range access.
If the document size would also be appended to the document, the partition would effectively become a double linked list and allow backward scanning from a known position (e.g. file end).

This would allow a couple of optimizations, most notably in order to detect a failed write the index would become irrelevant and hence allow recovery on a partition level (see also #31), because for that we need to scan the partition backwards and find the last finished commit and truncate up to that.
Possibly worth to note that a failed write will likely have an invalid "document size" at the end though, so likely it's not as trivial for this specific use-case.

[albe]

Resolved by #149