plugin/forward: Increase minimum read timeout to 200ms (#1889)

After several experiments at SoundCloud we found that the current
minimum read timeout of 10ms is too low. A single request against a
slow/unavailable authoritative server can cause all TCP connections to
get closed. We record a 50th percentile forward/proxy latency of <5ms,
and a 99th percentile latency of 60ms. Using a minimum timeout of 200ms
seems to be a fair trade-off between avoiding unnecessary high
connection churn and reacting to upstream failures in a timely manner.

This change also renames hcDuration to hcInterval to reflect its usage,
and removes the duplicated timeout constant to make code comprehension
easier.

plugin/forward/proxy_test.go

@@ -27,7 +27,7 @@ func TestProxyClose(t *testing.T) {
 
 	for i := 0; i < 100; i++ {
 		p := NewProxy(s.Addr, nil)
-		p.start(hcDuration)
+		p.start(hcInterval)
 
 		go func() { p.Connect(ctx, state, false, false) }()
 		go func() { p.Connect(ctx, state, true, false) }()