The method starts a task that uploads all files from the given
keyspace's snapshot to the requested endpoint/bucket. The task runs in
the background, its task_id is returned from the method once it's
spawned and it should be used via /task_manager API to track the task
execution and completion (hint: it's good to have non-zero TTL value to
make sure fast backups don't finish before the caller manages to call
wait_task API).
If snapshot doesn't exist, nothing happens (FIXME, need to return back
an error in that case).
If endpoint is not configured locally, the API call resolves with
bad-request instantly.
Sstables components are scanned for all tables in the keyspace and are
uploaded into the /bucket/${cf_name}/${snapshot_name}/ path.
Task is not abortable (FIXME -- to be added) and doesn't really report
its progress other than running/done state (FIXME -- to be added too).
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>