evolve: add a push pass using bundle2 Instead of using a dedicated wireprotocol commands, we use bundle2 to transmit an obs marker parts. This aims at both testing bundle2 more and to limit the amount of special code we needs to put in simple for server to fit our needs. The massive test changes comes from the fact we can use this fast path for both remote and local push.

evolve: make exchange output compact in the old pushkey case too.

evolve: compatibility with memfilectx change The future 3.1 have have more sophisticated memfilectx object. We add a compatibility layer for them.