Verify that startSync skips header download and directly requests
blocks when the header chain is already caught up to the peer's
height but the block chain lags behind.
Refactor fetchHeaderBlocks and buildBlockRequest to take an explicit
peer parameter instead of implicitly using sm.syncPeer. This makes the
caller responsible for choosing which peer to fetch from and adds a nil
guard to prevent a panic if the sync peer has been cleared.
headers
We introduce buildBlockRequest that'll create a getdata message based
off of the block index instead of the headerList in SyncManager.
The new fetchHeaderBlocks utilizes buildBlockRequest and now will create
fetch requests based on the processed block headers.
isInIBDMode returns if the SyncManager needs to download blocks and sync
to the latest chain tip.
It determines if it's in ibd mode by checking if the blockchain thinks
we're current and if we don't have peers that are at higher advertised
blocks.
