Replace -proxy-mode=<bool> with -proxy=<enum>

The proxy flag really needs three values, not just true/false. Establish the flag as an enum now rather than after v0.5-rc is cut to prevent unnecessary churn. Once the `activeSize` cluster config is actually supported, we can implement the "fallback" proxy mode.
Ah, using it like a set. I guess we could do that. That would make the lookup a little simpler but it would make printing the values with `strings.Join()` a lot more complicated. I feel like its simpler to just leave it.