no peers available, disabling sync (finally explained, inbound vs outbound inconsistency)

[antiochp]

Thanks to @bladedoyle I think we tracked down one scenario where we see the following regularly in the logs -

20201211 16:04:08.807 WARN grin_servers::grin::sync::syncer - sync: no peers available, disabling sync

When syncing we find "max diff" from all our peers (inbound and outbound) then we randomly select an outbound peer with that diff to sync from.
But we can be in a situation where "max diff" is from an inbound peer without any outbound peers currently being at that diff (say a new block was very recently found) and our peers had not yet updated local diff via ping/pong (every 5-10secs).

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We only want to sync from peers we know we reached out to.

Need to think through how best we can approach this.

[antiochp]

This affects all stages of sync -

• header sync
• state sync (txhashset.zip)
• body sync (full blocks)

Behavior here did change recently to make this consistent throughout the sync process.
We were originally inconsistent with how we handled the peer filtering/selection throughout the sync process #3458