storage: ClientImpl interface now includes Close

This is now necessary because bolt doesn't allow multiple DB handles, so the bolt piece completion DB instance must be at the client level. The new method is not currently used from the Client however, this maybe necessary to avoid leaks.

storage/interface.go

@@ -9,6 +9,7 @@ import (
 // Represents data storage for an unspecified torrent.
 type ClientImpl interface {
 	OpenTorrent(info *metainfo.Info, infoHash metainfo.Hash) (TorrentImpl, error)
+	Close() error
 }
 
 // Data storage bound to a torrent.