feat(swingset): boot xs-worker vats using snapshots from snapstore

[dckc]

This is an initial snapstore API integrated with the xs-worker vat manager factory.
In rough measurements over the swingset tests, it saves 40% over running without snapshots (2:29 vs. 3:59).

XXX enhances test:xs-worker to run all swingset tests with SWINGSET_WORKER_TYPE=xs-worker. 3 tests seem to fail, currently:

1. device-bridge › bridge device can return undefined
2. metering › dynamic-vat-unmetered › unmetered dynamic vat
3. metering › within-vat › metering within a vat - failed to setBundle: (an object)

Recall that test:xs is currently option, so these don't block merges.

ref #2273

I'd like to say "fixes..." but #2273 includes discussion of kernel DB state, taking snapshots after the initial supervisor bootstrap, etc.

[dckc]

Ouch... the size of the compressed snapshot is different between nodejs 12.x and 14.x. Maybe ziplib was refined?

The sha512 of the uncompressed snapshot didn't change, though... is that close enough, @warner ?

[dckc]

@warner please take a look.

Note: to get all the tests to pass, I had to tolerate different size compression results that seem to be due to different node versions: 5ac3db1

[warner]

Ouch... the size of the compressed snapshot is different between nodejs 12.x and 14.x. Maybe ziplib was refined?

The sha512 of the uncompressed snapshot didn't change, though... is that close enough, @warner ?

Yeah, let's define the snapshot ID as the hash of the uncompressed file. That's not quite as convenient as hashing the compressed version, but it's more appropriate, and would let us change the compression algorithm in the future without inadvertently changing the IDs.

[dckc]

fileHash wasn't right and fixing it seems to reveal that xsnap is not deterministic

p.s. for follow-up, see #2776

[dckc]

@warner I brought this back up-to-date.

No callers of makeSwingsetController are setting the snapstorePath option yet, so it's only exercised in testing.

[dckc]

ok, @warner @FUDCo I'm finally getting the ci results I expect. Please take a look.

[FUDCo]

Very nice.