make vat-localchain resumable

[turadg]

What is the Problem Being Solved?

The localchain vat is upgradable, but when it does any outstanding promises will be severed.

Description of the Design

• Change the type of every return to PromiseVow (@returns {Promise<T>} becomes @returns {PromiseVow<T>} to ensure that callers of that function use either when or watch to extract T)
• Use watch for the deposit method implementation (but I think we can still consider skipping that one)

Security Considerations

Scaling Considerations

Test Plan

Upgrade Considerations

[mhofman]

To clarify, the localchain vat seem to be resumable today given the following:

• We maintain the scheduling property where a vat is only upgraded after swingset is quiescent (assumptions about current scheduling and how upgrades are triggered)
• deposit() deals with a "local to the chain" payment object (assumption about origin of issuers / payments).

Since we'll likely need to stop making these assumptions in the future, consumer of the localchain API should be able to handle a vow return value. Changing the type now helps detect cases where consumers are not ready.