refactor DbSlice::ExpireParams

[BorysTheDev]

we have several fields in DbSlice::ExpireParams: int64_t value = INT64_MIN; // undefined TimeUnit unit = TimeUnit::SEC; bool absolute = false;

All these fields can be exchanged with one int64_t ms_timestamp, that will be created in the constructor or method.

[jolneetanky]

Hi, I'd like to work on this issue as my first contribution to DragonflyDB!