ChainSyncServer: report a tip consistent with the instruction

When the ChainSyncServer sends an instruction to a client, it also includes the tip of the current chain. However, this tip is currently not guaranteed to be consistent with the instruction:
https://github.com/input-output-hk/ouroboros-network/blob/6ca4a5262f999f7624aee61074203d9e735ce12b/ouroboros-consensus/src/Ouroboros/Consensus/MiniProtocol/ChainSync/Server.hs#L103-L105

Quoting @dcoutts:
> [this could lead to] e.g. [a] situation where we tell a client to roll forward, but the node is concurrently rolling back and the tip we report has an earlier block number than the block or header we're serving. It's just odd. Not that things should be relying on it much.