`PendingMgsUpdate` slot id should be a u16

Currently, the slot id (which identifies a cubby number, switch number, etc.) is:

- a u32 in MGS
- a u16 in inventory
- a u32 in `PendingMgsUpdate`

It seems like we should use a `u16` everywhere.  It's hard to imagine more than 65K cubbies in one rack.