Allow upgradesstables to be run against a specified directory

Currently, upgradesstables only modifies live SSTables. Because sstableloader cannot stream old SSTable formats, this makes it difficult to restore data from a snapshot taken in a previous major version of Cassandra.

Allowing the user to specify a directory for upgradesstables would resolve this, but it may also be nice to upgrade SSTables in snapshot directories automatically or with a separate flag.