obsolete: experimental flag to get debug about obsmarkers exchange The obsolescence markers exchange is still experimental. We (developer) need more information about what is going on. I'm adding an experimental flag to add display the amount of data exchanged during bundle2 exchanges.

bundle2: add an 'idx' argument to the 'getbundle2partsgenerator' This argument let extensions control in what order bundle2 part are generated server side during a pull. This is useful to ensure the transaction is in a proper state before some actions or hooks happens.