FluxDB: Should implement the graceful shutdown pattern

[jubeless]

FluxDB server at the moment simply waits 5 seconds on shutdown before close the HTTP server. It should implement a GracefulShutdown with the derr library to allow a delay before actually shutting down

[maoueh]

Actually fluxdb needs to move out of using derr.IsShuttingDown and instead rely on the shutter.Shutter terminating to make its decision.