obsolete: append new markers to obsstore file instead of rewriting everything
This is the second step toward incremental writing of marker inside a
transaction. The obsstore file is now handled append only.
Header writing have been extracted from _writemarkers.
Because the _writemarkers method have been dropped, the push code
directly reuse the serialised content of local repo `listkeys`. This
is not very pretty, but this part of the protocol still need major
improvement anyway.