Allow to use several backends with a RPCServerApp
This is needed for the swh-counters module, one backend uses Redis to manage the counters by themselves, and another one manages the history files using prometheus. Each one can possibly evolve separately or can use a different backend implementation one day or another.
Related to swh/infra/sysadm-environment#3165 (closed)
Migrated from D5428 (view on Phabricator)