feat: use xsnap worker CPU meter and start reporting consumption

[michaelfig]

Refs #2322

We implement the same basic policy as in the local worker: refill the meter with a constant value (1e7 computrons) before every crank and terminate the vat if it consumes more than that in a crank.

There is still a long way to go, but at least now we detect and terminate an xsnap vat upon infinite loops.

[dckc]

I'm hesitant to move forward with -DmxMetering=1 while Moddable considers it experimental. I'd like to hear from @dtribble on that, but presuming you're already in communication with him about it, I approve.

I'd really rather not have any in the type annotations, but I don't suppose that's critical.

[dckc]

I think I should withdraw my approval until we have clarity on @warner 's question:

shouldn't the meterStats be part of the OK that comes back from the worker?

Need another pass due to important changes.

[dckc]

I was about to approve this, but then ava hangs when I yarn test because some xsnap process is hanging around.

[dckc]

... ava hangs when I yarn test because some xsnap process is hanging around.

diagnosis: makefiles/lin/xsnap.mk is missing -DmxMetering=1

How did this get thru ci?

eek! packages/xsnap doesn't seem to get its tests run in ci: https://github.com/Agoric/agoric-sdk/pull/2384/checks?check_run_id=1875722804

[dckc]

makefiles/lin/xsnap.mk is missing -DmxMetering=1

and the test-quick ci job is missing packages/xsnap altogether!

[warner]

Tentative approval, please check the comments and see if the recommended changes are ones you're comfortable with. The XSNAP_VERSION thing is the one I'm most concerned about. The message-result protocol is the one I thought the most about but it might not be worth doing anything about.

[michaelfig]

... ava hangs when I yarn test because some xsnap process is hanging around.

diagnosis: makefiles/lin/xsnap.mk is missing -DmxMetering=1

I've added preflight checks to xsnap.c to reject metering unless -DmxMetering.

How did this get thru ci?

eek! packages/xsnap doesn't seem to get its tests run in ci: https://github.com/Agoric/agoric-sdk/pull/2384/checks?check_run_id=1875722804

Added!