exchange: add a `buildobsmarkerpart` function We'll have to build an obsmarker part for both push and pull. So we build a function to factor out the common part.

obsolete: add a `commonversion` function This function returns the highest common version between the locally known formats and a list of remotely known formats. This is going to be useful to know what format should be used for exchange.

bundle2: add a `obsmarkersversion` function to extract supported version Right next to the function that encodes the supported versions in capabilities we add a function that decodes the versions out of capabilities. This is going to be useful to know what formats can be used for exchange.