* Refactor so that RetryLimit::Future is std::marker::Sync
* Make the syncer future std::marker::Send by spawning tips futures
* Download synced blocks in chain order, not HashSet order
* Improve MockService failure messages
* Add closure-based responses to the MockService API
* Move MockChainTip to zebra-chain
* Add a MockChainTipSender type alias
* Support MockChainTip in ChainSync and its downloader
* Add syncer tests for obtain tips, extend tips, and wrong block hashes
* Add block too high tests for obtain tips and extend tips
* Add syncer tests for duplicate FindBlocks response hashes
* Allow longer request delays for mocked services in syncer tests