NN should update storageMap on first heartbeat

Currently the NN updates DatanodeDescriptor#storageMap while processing block reports but not from heartbeats. This results in the DatanodeStorageInfo#blockContentsStale being stale even after the first heartbeat is received.

  void receivedBlockReport() {
    if (heartbeatedSinceFailover) {
      blockContentsStale = false;

We need to update storageMap if we see a new storage ID in a heartbeart.