Cold Storage Directory Support for DTCS

Date Tiered Compaction for time series data at high volume it seems like a good candidate for multi-tiered storage options for cost reduction purposes.

For example setting max_sstable_age_days = 1 means that once a file hits 1 day old it won't be a candidate for compaction any longer. For immutable time series data you could in theory then have a new DTCS and yaml setting that says move anything over X days (table def) into the cold storage directory from the yaml file.

This way you could have pricer, smaller SSD drives for recent data and cheaper, magnetic storage for longer term data that is infrequently read after a certain period of time.

cc Bj0rn for input and feasibility.