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/policy_test.go

@@ -5,6 +5,7 @@ import (
 	"net/http/httptest"
 	"os"
 	"testing"
+	"time"
 )
 
 var workableServer *httptest.Server
@@ -54,7 +55,7 @@ func TestRoundRobinPolicy(t *testing.T) {
 		t.Error("Expected second round robin host to be third host in the pool.")
 	}
 	// mark host as down
-	pool[0].Unhealthy = true
+	pool[0].OkUntil = time.Unix(0, 0)
 	h = rrPolicy.Select(pool)
 	if h != pool[1] {
 		t.Error("Expected third round robin host to be first host in the pool.")