add basic S3-Compatible Object Storage support

we have now an interface for filesystem backeds, this make easy to add
new filesystem backends

sftpd/sftpd.go

@@ -307,7 +307,7 @@ func GetConnectionsStats() []ConnectionStatus {
 					StartTime:     utils.GetTimeAsMsSinceEpoch(t.start),
 					Size:          size,
 					LastActivity:  utils.GetTimeAsMsSinceEpoch(t.lastActivity),
-					Path:          c.User.GetRelativePath(t.path),
+					Path:          c.fs.GetRelativePath(t.path, c.User.GetHomeDir()),
 				}
 				conn.Transfers = append(conn.Transfers, connTransfer)
 			}