tiasync: choose best state sync snapshot from multiple upstream peers

[misko9]

Currently, tiasync only supports a single upstream peer for state sync. If there are multiple upstream peers, the first connected upstream peer is used.

This enhancement will query all upstream peers for their snapshots and deliver the best snapshot to the fullnode.

Depending on tiasync's discovery time, the fullnode's discovery time may expire before the best snapshot is delivered. Especially due to tiasync's start latency. This may be okay if the fullnode just retries, but something to note and test.