multiplexer: Check backends with respect to their read-only nature

Prior to this commit, a multiplexer instance would start during its initialization to check the configuration of all its backends but also check whether they can write or not even when not needed. As it exists read-only backends for a multiplexer, that would just fail to start the service.

This evolves the objstorage to grow a check_write attribute (hence parametric through configuration file). And then adapts accordingly the multiplexer constructor so it can check the configuration of its backends according to their respective configuration.

Refs. #4746 (closed) Refs. swh/infra/sysadm-environment#5334 (closed)