obsolete: move _encodemarkers next to _readmarkers

obsstore: store and preserve ondisk version

obsolete: have _readmarkers return the format version readmarkers is not returning the version of the format it read from. This will let callers know about the format and allow them to preserve it.

obsolete: support for any known obsstore format when reading or writing We can now read and write any known format. The list of known formats currently has one element (0). The obsstore itself is not aware of multiple formats yet and always uses format 0.

obsolete: move _fm0encodeonemarker next to _fm0readmarkers

obsolete: rename _encodeonemarker to _fm0encodeonemarkers This function encodes markers in version 0 of the format.

obsolete: extract the part of _readmarkers specific to format version 0 If we are to introduce new formats we need to be able call different functions for different formats. Creating a function for format version 0 is the first step.

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.

obsstore: have the `mergemarkers` method return the number of new markers The mergemarkers function now returns the number of unknown markers in the stream that have been added to the obsstore. This is similar to what `obsstore.add` already does. The method gains a docstring in the process.