Improve the experience of using the Master webpage by change the loading process of snapshot list to asynchronous

Background: When there are many snapshots, loading the master webpage is very slow, which affects the experience. (Our cluster has more than 3000  snapshots, and it takes about 10 seconds to load the master webpage each time)

1. The snapshot list is not in the master memory, hdfs need to be scanned when loading

2. Changing the process of loading  snapshots to asynchronous can improve the experience