Applicationset-controller overloads api-server and have memory leak

[jgwest]

Fixes #153

This was caused by our creating a SettingsManager (SettingsManager in settings.go in Argo CD) with every call of the ClusterGenerator. Since the settings manager contains a client and 2 listers, each instance was opening sockets and multiple goroutines to handle resource caching with the k8s server. The socket and these connections were what were occupying the leaked memory:

# From SettingsManager
	clientset  kubernetes.Interface
	secrets    v1listers.SecretLister
	configmaps v1listers.ConfigMapLister

The fix is:

• Create a single instance of SettingsManager (which is what Argo CD does), and query that instance on each cluster generator call.
• I've also added Argo's stat logger, which outputs the memory usage and # of goroutines every 10 minutes, which should allow us to catch similar issues in the future.

[wtam2018]

LGTM
thanks @jgwest !