* Add a new message: BlockLocatorHighestHash.
* Add a new message: IBDBlockLocator.
* Implement HandleIBDBlockLocator.
* Reimplement findHighestSharedBlockHash.
* Make HandleIBDBlockLocator only return hashes that are in the selected parent chain of the target hash.
* Increase the cache sizes of blockRelationStore, reachabilityDataStore, and ghostdagDataStore.
* Fix wrong initial highHash in findHighestSharedBlockHash.
* Make go vet happy.
* Protect against receiving wrong messages when expecting MsgIBDBlockLocatorHighestHash.