This commit adds two commands:
pilosactl backup
pilosactl restore
The `Client` implementations have also been added to support these
commands:
func (c *Client) BackupTo(w io.Writer, db, frame string) error
func (c *Client) RestoreFrom(r io.Reader, db, frame string) error
Backups occur on a per-frame basis and all slices from the cluster
are packed into a single tar file. The backup tool attempts to
read from owner nodes in a random order and will retry against
the next owner if one fails.
During restore, the slices are restored to all owner nodes. This
allows users to backup a frame from one cluster and restore it
to a different one -- even if the topology of the new cluster is
different.
