* 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.
* 1) Calculate pruning point incrementally
2) Add IsValidPruningPoint to pruning manager and consensus
3) Use reachability children for selected child iterator
* Add request IBD root hash flow
* Fix UpdatePruningPointByVirtual and IsValidPruningPoint
* Regenerate messages.pb.go
* Make the pruning point the earliest chain block with finality interval higher than the previous pruning point
* Fix merge errors
