Improve decoding for NaiveDateTime in RowBinary

[ruslandoga]

# current — allocates a DateTime only to immediately discard it
dt = DateTime.from_unix!(s)
dt = case timezone do
  nil -> DateTime.to_naive(dt)  # ← throws away the DateTime
  ...
end

# faster — go direct
nil -> NaiveDateTime.from_gregorian_seconds(s + @epoch_gregorian_seconds)