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.
Btw this doesn't mean `proxyFlagValues` is a const now, since a string is a []byte. Is that what you were shooting for?