* Rename BestTipHeight so it can be generalised to ChainTipSender
`fastmod BestTipHeight ChainTipSender zebra*`
For senders:
`fastmod best_tip_height chain_tip_sender zebra*`
For receivers:
`fastmod best_tip_height chain_tip_receiver zebra*`
* Rename best_tip_height module to chain_tip
* Wrap the chain tip watch channel in a ChainTipReceiver type
* Create a ChainTip trait to avoid tricky crate dependencies
And add convenience impls for optional and empty chain tips.
* Use the ChainTip trait in zebra-network
* Replace `Option<ChainTip>` with `NoChainTip`
Co-authored-by: Janito Vaqueiro Ferreira Filho <janito.vff@gmail.com>
Co-authored-by: Janito Vaqueiro Ferreira Filho <janito.vff@gmail.com>
teor