Use BIP-130 `sendheaders`

[rustaceanrob]

Describe the enhancement

Before BIP-130, new blocks are announced via an inv message. This is currently how the node discovers new blocks. To save a round-trip of inv -> getheaders -> headers, BIP-130 allows for block announcements by simply sending a headers message as opposed to an inv. With the changes to chain, the node should now be able to implement BIP-130.

ref: https://github.com/bitcoin/bips/blob/master/bip-0130.mediawiki

Additional context

• Any pending CFHeader messages should be reset upon receiving headers (previously done when receiving inv)
• The state should be reset to Behind if the node receives a headers message (previously done when receiving inv)
• The sendheaders message must be sent before sending verack

[rustaceanrob]

This one subtly touches a lot of the repository considering the node state, pending messages, etc. There are a number of refactors I want to do to build up to this one.