Kill heartbeat if layer has finished processing
Kill heartbeat if layer has finished processing

Fixes a bug where the heartbeat would mark the layer status as failed
even if the layer status had already been marked as COMPLETED.

Connects #270

azavea/raster-foundry,azavea/raster-foundry,kdeloach/raster-foundry,kdeloach/raster-foundry,raster-foundry/raster-foundry,azavea/raster-foundry,raster-foundry/raster-foundry,aaronxsu/raster-foundry,kdeloach/raster-foundry,raster-foundry/raster-foundry,aaronxsu/raster-foundry,kdeloach/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,kdeloach/raster-foundry,azavea/raster-foundry,aaronxsu/raster-foundry