evolve: store the obsmarkers to be deleted in evolvestate
Sometimes, we don't create a new node and obsolete one of the old node in favour
of an existing node. When user calls, `hg evolve --abort`, we need to strip that
obsmarker to go back in the same old state. The obsmarker created for new node
are stripped when the new nodes are stripped.
This patch start storing such obsmarkers in the evolvestate so that we can delete
them later if required. Right now we just store successsor and predecessor
information, we might need a better obsmarker serialization techinque in future
to make it more robust.