why:
  Same as `kMap[$1]`

why:
  So it is directly comparable to `kMap[$1]`

why:
  Then a separate `FilterRef` type is not needed, anymore

why:
  New name more appropriate.

why:
  This allows to avoid copying into the `balancer` (see next patch set)
  most of the time. Typically, only one instance is running on the backend
  and the `balancer` is only used as a stage before saving data.

why:
  Avoid useless copy when staging `top` layer for persistently saving to
  backend.