app/vminsert/netstorage: increase timeout for pushing data from vminsert to vmstorage by 3x

Our clients report that the previous timeout could lead to frequent errors when
vmstorage starts background merge for big parts on slow HDD.

app/vminsert/netstorage/netstorage.go

@@ -117,7 +117,7 @@ func (sn *storageNode) sendBufLocked(buf []byte) error {
 			return fmt.Errorf("cannot dial %q: %s", sn.dialer.Addr(), err)
 		}
 	}
-	timeoutSeconds := len(buf) / 1e6
+	timeoutSeconds := len(buf) / 3e5
 	if timeoutSeconds < 60 {
 		timeoutSeconds = 60
 	}