support flux config reload to reconfigure fluxion

[grondo]

Flux supports opt-in configuration reload for modules. When flux config reload is used, the broker re-reads configuration and sends a <module>.config-reload RPC to all modules. The new config can be read from the request via flux_config_reload_decode (msg, &conf). The updated config can then be unpacked and supported configuration updated on the fly.

Note: It was discussed in the meeting that it may be more tractable to support Fluxion restart with running jobs than a configuration reload.