change set-status RPC to allow specifying multiple paths

[jameshcorbett]

Problem: the set_status RPC defined in resource_match.cpp takes only a single vertex path. The flux-coral2 project sometimes needs to send bulk updates. Since there are only two states, UP and DOWN, I propose changing the RPC payloads from

{"resource_path": PATH, "status": "up"/"down"} to {"up": [PATH, PATH, ...], "down": [PATH, PATH, ...]}.