do not back off on timeouts when sending reports

...since doing so unnecessarily throttles report sending, to the point
where the app is receiving reports so infrequently that often it has
no data to show.

The timeout period itself is sufficient to prevent thrashing.

Fixes #2745.

probe/appclient/app_client.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"net"
 	"net/http"
 	"net/rpc"
 	"net/url"
@@ -215,7 +216,11 @@ func (c *appClient) doWithBackoff(msg string, f func() (bool, error)) {
 			backoff = initialBackoff
 			continue
 		}
-
+		if netErr, ok := err.(net.Error); ok && netErr.Timeout() {
+			log.Errorf("Error doing %s for %s: %v", msg, c.hostname, err)
+			backoff = initialBackoff
+			continue
+		}
 		log.Errorf("Error doing %s for %s, backing off %s: %v", msg, c.hostname, backoff, err)
 		select {
 		case <-time.After(backoff):