This patch adds
- Changes in sstables_loader::restore_tablets() method
It populates the system_distributed_keyspace.snapshot_sstables table
with the information read from the manifest
- Implementation of tablet_restore_task_impl::run() method
It emplaces a bunch of tablet migrations with "restore" kind
- Topology coordinator handling of tablet_transition_stage::restore
When seen, the coordinator calls RESTORE_TABLET RPC against all tablet
replicas
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>