obsolete: rename all _fm to _fm0 This change is because these formats are used for version 0 of the obsstore format. This is going to be useful in the future when we introduce new versions of the format.

obsolete: rename _fnodesize to _fmfnodesize All variables involved in the obsstore format are prefixed with `_fm`. `_fnodesize` was the exception. It is now back in line. This is meaningful as we'll need to distinguish between multiple versions of the obsstore format.