middleware/proxy: async health checks (#749)

* Switches out Unhealthy bool for OkUntil timestamp

* Make sure servers are healthy forever if there are no health checks

* Moves health check off into a go routine to avoid blocking conditions

* Improved logging info

* Fixes initial date

* Fixes health checking; alters tests to adapt to async health checking

* Moves future variable into static upstream and populates it in more places

* Restores silencing of stdout during testing

* Restores silencing of stdout during testing

* keeps check url string once built

* Removes debug message

* uses zero value to signal no checking; reduces in-mutex code to a fetch

middleware/proxy/grpc_test.go

@@ -27,6 +27,7 @@ func TestStartupShutdown(t *testing.T) {
 		Policy:      &Random{},
 		Spray:       nil,
 		FailTimeout: 10 * time.Second,
+		Future:      60 * time.Second,
 		MaxFails:    1,
 	}
 	g := newGrpcClient(nil, upstream)