conveyor should provide runtime settings for condenser

[sneak]

Condenser is configured two ways right now:

1. defaults in codebase
2. overrides in environment

I would like to add a 3) runtime variables provided by conveyor

Condenser would have a set of "runtime config" keys that have defaults loaded in from the codebase. On startup and every 30 seconds thereafter, condenser queries conveyor for a set of condenser runtime setting overrides, which conveyor probably wants to store in redis or postgres.

Watt could then be upgraded to make signed calls to conveyor to modify these runtime settings which would take effect in condenser in under a minute.

Some items that could go in here:

bool maintenance_mode
steem_username[] currently_featured_communities
steem_tag[] currently_featured_tags

et c

[sneak]

This could be used by more than just condenser, too.

[jnordberg]

Spec proposal:

set_service_config <service_name> <config_object>
get_service_config <service_name> returns configuration object as stored by above call

Both methods require an administrator signature.

Stored in s3 under conveyor_{{ service_name }}_config.json

[sneak]

the only change i would suggest here (open to other designs) is as follows:

the original design here is a sort of global k/v table, not just settings for condenser. the things that consume them can decide if certain keys in the global table apply to them. i imagine keys like condenser_read_only_mode or faucet_disable_signups or similar.

we also need to cache these in conveyor for e.g. 60s, because that's a lot of GET calls to S3 otherwise (with each process of each service asking conveyor for the runtime config twice a minute). if we do a backing file per-service, that multiplies that times the number of services (even with caching).

I imagine there will be at most like 100 keys in this file across all services, so i'm thinking we just call it conveyor_serviceconfig.json and not bother with the <service_name> part.

thoughts?