After connecting to network, user should send request for block headers

[tgoldenberg]

On initialization, the program should first look locally to see if block headers are stored.

If they are, they should be set and the last block header should be sent to the channel.

Once the last saved header is sent out, the program should await a response with block headers starting from the last one saved.

Once the program receives the block headers (a max of 50), it should set its "last block header" and reject incoming messages that are not set to it.

As the client accepts the block headers, it should validate their integrity. If any are not validated then the previous state of the chain should be accepted.

When the client receives a chunk of headers less than 50, it can send a verification request that the headers are up-to-date. The network can send a verification that this is true.