namenode can save images in parallel to all directories in fs.name.dir

The namenode restart times can be reduced if the namenode can save its image to multiple directories (specified in fs.name.dir) in parallel.

The NN has a 6 GB fsimage and 1 MB edits file. The NN needed 10 minutes to load fsimage/edits into memory. It needs 7 minutes to read the 6 GB image. There are two directories in fs.name.dir. it takes about 2 minute to save the image into first directory and another 2 minutes to save the image to the second directory.