Extends compaction for new resharding algorithm. Not wired yet.
New resharding will compact shared sstable(s) and create one
sstable for each owner. It's up to the caller to open these
new unshared sstables at their respective column families.
This new approach will save a lot of bandwidth because we'll
no longer read the entire shared sstable #smp::count times.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>