Global index

This adds a --global-index option to tablesnap, tablechop, and tableslurp. --global-index is primarily aimed at adding compatibility to cassandra's multi-datadir (JBOD) setup.

If your cassandra has multiple data-dirs, default tablesnap options will render PIT recovery and cleanup impossible. This is due to a single table being split over multiple dirs (sstables end up in a random dir from the list). The current json metadata format only accounts for files in the same dir as the currently processing sstable. This patch adds a new index format that keeps the state of all files in all target paths at the time of upload. This makes it possible to recreate the entire structure as it was during the backup of any file.