remove 'stats' from vatAdmin API

[warner]

What is the Problem Being Solved?

The vatAdmin~.createVatDynamically() function returns an "admin node" that lets the parent control the new child vat they just spawned. This admin node offers a small number of features:

• a done() promise that tells you when the vat has terminated
• a terminateWithFailure() function that lets you kill the vat early
• an adminData() function that returns "admin statistics" about the vat

The statistics consists of four counters, which measure the number of imported objects, promises, and devices, and the number of deliveries that have been made to the vat.

However, I don't think we've seen a use case for sharing these stats with userspace, and I want the underlying vatKeeper to collect more detailed data (including GC comings and goings), which I'm reluctant to reveal to userspace (and thus include as part of our consensus behavior, which might our options in the future).

So I'm proposing to remove this data from userspace access.

@Chris-Hibbert I think you implemented this originally.. have you encountered a use for the admin stats since then? Any opinions on removing it?

Description of the Design

The "admin node" would remove the adminData() method.

Security Considerations

Less code === more security.

Compatibility Considerations

Better to remove this before promising compatibility.

Test Plan

The unit tests that check for this field (which use `test/vat-admin/bootstrap.js) will have those checks removed. No negative tests will be added (i.e. ones that try to call the method and assert that a TypeError occurred instead of success).

[Chris-Hibbert]

I have not seen a use for the functionality.

Once you collect these new stats, where would they be accessible?

I have no indication that the stats I originally added are the right ones to expose to the contract instance creator, they were just the numbers I could easily find. It does seem like contract developers will want performance data so they can improve behavior. We'll have privileged access; will there be any stats others can examine?

If we've thought about it, and not come up with any beneficial uses, I don't have a problem dropping it.

[warner]

#3333 has the new stats I'm planning to add. They'd be available from the controller, and by looking at the on-disk database directly, but not from userspace. Developers would probably be best-off getting them from a block explorer application of some sort, rather than allowing a vat itself to retrieve them. One of those things you must get by standing outside the system, rather than being visible from inside.