refactor: from / into types conversion

1. Trait `From` for `traft::row::Entry` now accepts a value instead of a
   reference in order to avoid unnecessary clones. A caller makes a
   clone itself if necessary.

2. Trait `From` implementation for `traft::Message` now accepts a
   reference instead of a value. Anyway, it uses msgpack under the hood
   and doesn't affect performance.