Add thread local mechanism for aggregating file system storage stats

As discussed in HADOOP-13032, this is to add thread local mechanism for aggregating file system storage stats. This class will also be used in HADOOP-13031, which is to separate the distance-oriented rack-aware read bytes logic from FileSystemStorageStatistics to new DFSRackAwareStorageStatistics as it's DFS-specific. After this patch, the FileSystemStorageStatistics can live without the to-be-removed FileSystem$Statistics implementation.

A unit test should also be added.