Stuck in fast sync with no peers and validator set > 1

[ebuchman]

If the validator set is greater than 1, but we have no peers, we will be stuck in fast sync, even if we have more than 2/3 of the voting power.

This is easy to replicate. Run a node with tendermint node --proxy_app=persistent_kvstore. Add a new validator with 1/11 of the voting power: curl localhost:26657/broadcast_tx_sync?tx=\"val:0e783029302e8e4713f2c3085ee35c3335bbfd698f2bba09d3196aca3872d651/1\". Now restart the node and it will be stuck in fast sync.

We treat the special case of being the only validator in https://github.com/tendermint/tendermint/blob/develop/node/node.go#L236. We should generalize this - instead of just checking if we're the sole validator, we should check if we have > 2/3 of the voting power - if so, we can skip fast sync. If we have less than 2/3 of the voting power, we'll need at least one peer before we can switch out of fast sync.

Discovered this while trying to test that #2747 is backwards compatible by making validator set changes on current version of the software and then trying to run that branch.

[ebuchman]

Never mind - we don't necessarily want this. It's possible a node with +2/3 is still behind and wants to sync up. It's even possible if theres just a single validator. But we'll leave the heuristic for now and others can just turn off fast sync if they find themselves in this position.