Limit connection pool size on UDS: we should only need one per container (#1252)

Hopefully this reduces FD usage even further.

api/agent/agent.go

@@ -964,6 +964,8 @@ func (a *agent) runHot(ctx context.Context, call *call, tok ResourceToken, state
 
 		udsClient = http.Client{
 			Transport: &http.Transport{
+				MaxIdleConns:        1,
+				MaxIdleConnsPerHost: 1,
 				// XXX(reed): other settings ?
 				IdleConnTimeout: 1 * time.Second,
 				DialContext: func(ctx context.Context, _, _ string) (net.Conn, error) {