fine-grained control over data directories

Currently Cassandra supports multiple data directories but no way to control what sstables are placed where. Particularly for systems with mixed SSDs and rotational disks, it would be nice to pin frequently accessed columnfamilies to the SSDs.

Postgresql does this with tablespaces (http://www.postgresql.org/docs/9.0/static/manage-ag-tablespaces.html) but we should probably avoid using that name because of confusing similarity to "keyspaces."