Upgradable network vat

[FUDCo]

This is a first cut at making the network vat upgradable.

It also includes a refactoring that moves the network vat code from being in a mix of the SwingSet package and the vats package to being 100% in the vats package.

Currently it passes all the tests I could find that in any way implicate the network vat (in the vats, solo, cosmic-swingset, and pegasus packages). However, while it now keeps all of its state that I could identify in durable storage, there is as yet no test as to whether it actually achieves upgradability. As such, this PR should be viewed as a step on the road to achieving upgradability without yet making any assertions as to whether that destination has in fact been reached. Consequently I would leave the motivating issue (#5696) open for the time being.

I have made an unsuccessful first cut at attempting to test upgradability, but various complications with the current network vat tests (most notably that these tests don't actually run in a swingset) have meant that while the proposed testing strategy that @michaelfig pointed me at seems to be sound (basically, a network vat restart in the middle of the test network - ibc test), the existing tests themselves do not lend themselves to being adapted in this way. This is work for the future but I believe we have concluded that it is not on the current critical path.

[michaelfig]

Sorry, it looks like I didn't approve in time to avoid bitrot. Chip, would you be able to pick this up again if you could update it to current master? I'll gladly review this time.

[warner]

Incidentally, it sounds like we don't have plans to include dynamic IBC in the Vaults release, therefore we won't be including a vat-network in the bulldozer upgrade. The bundle might be included anyway, by virtue of being in decentral-core-config.json, except that we probably won't use that config file (we'd use something like decentral-main-psm instead). cc @dckc

[turadg]

Subsumed by #8721