Slow down DNS poll interval

There's no reason to expect anyone is changing their DNS that fast,
and the probe queries all endpoints every time it polls so slowing
down will reduce effort.

probe/appclient/resolver.go

@@ -14,7 +14,7 @@ import (
 )
 
 const (
-	dnsPollInterval = 10 * time.Second
+	dnsPollInterval = 5 * time.Minute
 )
 
 // fastStartTicker is a ticker that 'ramps up' from 1 sec to duration.