Remove unnecessary promiseRegistrations

[mhofman]

What is the Problem Being Solved?

While investigating the growing heap of v43-walletFactory (#10706), we identified then worked around a liveslots/virtualReferenceManager leak through watchPromise's promiseRegistrations. While we did the minimal possible fix in #10758 and plan to fix virtual reference leak in #10757, it appears that we don't need the virtual promiseRegistrations Map in the first place.

#5514 introduced the scalar map to make sure the promise would stay in slotToVal even if it wasn't exported. However 1) I believe that it wasn't actually needed since we'd never have collected it anyway (part of the bug we want to fix in #10757) and 2) we've since changed the logic to make sure the promise is exported if locally decided. As such the promise will maintain its vpid association while the kernel pillar is maintained, aka until it settles (either locally or remotely). And even then, the settle handler doesn't seem to be relying on the mapping of the promise object to vpid anyway since it closes over the vpid and doesn't use the promise identity at all.

Description of the Design

Replace promiseRegistrations scalar MapStore by a heap Set used only to keep track of promises watched during buildRootObject before loadWatchedPromiseTable is called (to avoid reviving promises newly watched)

Security Considerations

None

Scaling Considerations

Removes a bunch of unnecessary virtual data syscalls.

Test Plan

Existing test coverage should be sufficient. Make sure it covers promises watched and possibly settled before buildRootObject completes.

Upgrade Considerations

Liveslots code change so it requires a chain software upgrade and an upgrade of vats using watchPromise.