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.
I went back and forth on this. I want to document what it *will* be, but maybe the code isn't the right place to do that. I'll go ahead and remove it for now with the understanding that it will come back before v0.5-rc.