Add configuration key to automatically flush redis storage on startup

[philou-078]

Especially during development and tuning, I regularly need to flush the Redis storage before restarting the gas station instance.

This would bring a simplification/automatization if a configuration key allowed to performed it automatically when the gas station starts.

For example:

storage-config:
redis:
redis_url: ...
flushall_on_startup: true/false # false by default