RPC endpoint system_peers stuck at 0 best block number for 1 of 2 peers

[qdm12]

To reproduce, git checkout qdm12/end-to-end/rpc/system_peers. This branch contains a single commit wip adding a check verifying all peers have the same block numbers when querying the system_peers endpoint.
This tend to fail if we sleep for a few seconds (time.Sleep(5 * time.Second)), where one of the two peers connected is stuck on the best block number 0 (but never both, oddly).
The corresponding RPC handler is Peers() []common.PeerInfo but I did not find anything wrong there.

I'm not sure of the severity and depth of this bug, but I cannot 'quick-fix' it in #2470 hence the reason for this issue. Note this was found by adding more testing depth in #2470 but this wasn't asserted before either.