feat(cosmic-swingset): use compute meter to decide where blocks end

[dckc]

lightly tested

fixes #3582

tasks:

• vatCreated() how long does vat creation take?

[dckc]

note from discussion with @warner :

      // TODO: vatCreated() how long does vat creation take?
      // in particular, contract vats (which is probably all of them)
      // aim for <= 3sec of create-vat work in a block
      // e.g. 3 x 1 sec

[warner]

SyntaxError: The requested module '@agoric/swingset-vat' does not provide an export named 'computronCounter'

Let's copy that file into cosmic-swingset, I don't think I want SwingSet/ to be exporting it.

[dckc]

@michaelfig @warner please take a look. What do you think?

p.s. I plan to squash; the 3 separate commits are not worth keeping.

[michaelfig]

LGTM, modulo a slight refactor (which needs more than just accepting the suggestions I made).