Add rate limit cache and fixes

This change adds a loop that keeps a cache of credentials rate limits
as reported by the github API. The cache is updated every 30 seconds
and is purely informational for the user.

This change also adds some caching improvements. Functions that return
values from the cache as lists, will now sort by ID or creation date.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

cache/instance_cache.go

@@ -53,6 +53,7 @@ func (i *InstanceCache) GetAllInstances() []params.Instance {
 	for _, instance := range i.cache {
 		instances = append(instances, instance)
 	}
+	sortByCreationDate(instances)
 	return instances
 }
 
@@ -66,6 +67,7 @@ func (i *InstanceCache) GetInstancesForPool(poolID string) []params.Instance {
 			filteredInstances = append(filteredInstances, instance)
 		}
 	}
+	sortByCreationDate(filteredInstances)
 	return filteredInstances
 }
 
@@ -79,6 +81,7 @@ func (i *InstanceCache) GetInstancesForScaleSet(scaleSetID uint) []params.Instan
 			filteredInstances = append(filteredInstances, instance)
 		}
 	}
+	sortByCreationDate(filteredInstances)
 	return filteredInstances
 }