Chunk server: implement storage tiers configuration, by adding parameter "chunkServer.storageTierPrefixes", and interpret it as space separated pairs of chunk directory prefix followed by the corresponding tier. Allow to set / reconfigure storage tiers from the meta server.
Chunk server: set space available to -1 in chunk directory constructor to make SetStorageTiers() work on start up. Change SetStorageTiers() to make default tier assignment in the case if tiers prefixes is empty.