middleware/proxy: make it scale (#287)

* middleware/proxy

Use connection pooling for communicating with an upstream, instead of
opening a new socket every time.

This makes the proxy more efficient and allowed for some cleanups.

* Some cleanups

* Some fixes

* more

* Kill pool

* Add nil check

* remove pool

middleware/proxy/upstream.go

@@ -89,6 +89,7 @@ func NewStaticUpstreams(c *caddyfile.Dispenser) ([]Upstream, error) {
 				Fails:       0,
 				FailTimeout: upstream.FailTimeout,
 				Unhealthy:   false,
+
 				CheckDown: func(upstream *staticUpstream) UpstreamHostDownFunc {
 					return func(uh *UpstreamHost) bool {
 						if uh.Unhealthy {