Allow Foundation subsidy to be waived

[lukechampine]

v2 transactions have a NewFoundationAddress field, which, if present, updates both the primary and failsafe addresses. This PR tweaks the update logic slightly: if the new address is types.VoidAddress, only the primary address is updated. (State).FoundationSubsidy also now returns false if the primary address is types.VoidAddress. Together, these changes allow the Foundation to effectively waive the subsidy until a new, non-void primary address is set. This is preferable to receiving and then later burning the subsidy; instead, the subsidy is simply not created in the first place.

I also renamed "primary" and "failsafe" to "subsidy" and "management" to reflect the function of these addresses in v2. The "subsidy" address receives the subsidies, while the "management" address is used to update addresses. (Note that, thanks to spend policies, the old "primary" and "failsafe" can be combined into one address using a threshold policy.)

Checking for types.VoidAddress might seem like a bit of a hack. Alternatively, we could expand the definition of NewFoundationAddress. Technically, this would change the encoding of V2Transaction; however, AFAICT, it would not actually cause any breakage, as the field is a pointer and has never been used in the v2 testnet (hence always encoded as false).

[n8maninger]

🧪

[lukechampine]

Would be cool if there was a GitHub action that enforced that a test failed prior to the PR 🤔