add cache save/load

Since mixing 0.7 nodes with 0.6 is looking increasingly unlikely to be supported because of the deep changes to the Thrift API, we should allow saving out the 0.6 cache and loading it on startup so that we don't inflict the pain of an entire cluster of cold cache on upgraders.

The cache format should just be a list of row keys. Loading it is as simple as calling getColumnFamily (with a zero-column predicate) on each row, for row cache.

Key cache is more complicated, but only a little. First is that you have to de-duplicate the row keys from multiple sstables. (Saving which sstable version it's associated with is less useful, since that will be obsoleted by compaction.) Second is that we don't need to actually read any row data, we just need to go through the index locator part of the read path (getPosition).