blockmanager: once we're at tip, start to use sendheaders to learn of new chains

[Roasbeef]

Currently we rely on inv's at the chain tip to learn of new blocks. However, with BIP 130 nodes can opt in to instead receive a headers message once the main chain is being extended. We can take advantage of this to eliminate the extra set of round trips involved to fetch the new set of headers. Instead, we can learn of the new tip almost instantly.