liveslots may override vpid in eventual send handler

[mhofman]

Describe the bug

While investigating solutions for #10757, and how liveslots and the virtual reference manager handling of promises interacts, we realized that if an eventual send promise is added to virtual data before the liveslots handler for the eventual send is invoked in the next turn, then the handler will unconditionally allocate a vpid for the promise, overriding the one that would have been created when marshalling the promise in vdata.

To Reproduce

const p = E(foo).bar();
virtualSet.add(p); // allocates vpid for vdata serialization
await null; // liveslots handler unconditionally allocates new vpid

Expected behavior

The vpid allocated by vdata should be used by liveslots