Add reservation expiration time to manifest on per-reservation basis

[ibaldin]

With slice modify, users can be confused about when a particular reservation times out. 'Slice reservation time' really no longer makes sense (never did, really). We should take that out and replace with per-reservation expiration time in manifest.

Extend operations are really per-reservation operations anyhow.

We can then modify Flukes and client libraries to support this.

@YufengXin @paul-ruth

[ibaldin]

We can keep the semantics of the extendSlice call as is for now (as in 'extend all currently active reservations in this slice until date D', however long term we may want to fully expose the per-reservation semantics.