discovery: hook on the official discovery for obsmarker This will let the discovery be done once and used by core.

discovery: use a different setup step for discovery This will ease the dropping of the other part later.

evolve: remove unused drop random

obshash: resist to unknown revision during discovery

code movement: gather discovery code together The discovery code is the one thing that will survive the great purge.

bundle2: drop custom compat We'll rely on the core implementation to use bundle2.